I read a lot of fan translated manga. While not legal, it's often the only version of a given manga available in English.
For example, although anime exists for Birdy the Mighty, nobody except fans have ever translated the long-running manga (which has more than one incarnation, even in Japan it moved from one publisher to another. The current version is Tetsuwan Birdy Evolution.) I could argue that the series gets bogged down in minor details and plot points and never seems to go anywhere... which might be a reason why it hasn't been translated. But I still like reading it.
Another manga that's internet-popular but unlikely to be worth a professional translation is Only Sense Online. It's a slice-of-life kind of story that deals with the minutiae of crafting in an online world -- so similarly to Birdy, it gets bogged down in details a lot. The plot, what little there is, advances very slowly. It's not earth-shattering stuff, but it's still entertaining to read.
There are a lot of smaller manga that I really like that would never make financial sense to translate: Tadashi Ore wa Heroine Toshite, Hungry Marie, Cylcia = Code, and Magical Trans! just to name a few. I don't expect to ever see a professional translation of these, but I've been able to read them because fans translated them.
When it comes to more popular series however, there's usually a professional translation, and you're supposed to support the company doing the translating and buy the volumes they publish. After all, the only thing that guarantees new stuff will be translated and published is if they make money at what they do. I feel like I'm increasingly in the minority when it comes to wanting physical books in my hands -- and I admit, my house is crammed full of books, and I don't need more (I'll have to move them all about this time next year), but even though I've more-or-less converted to digital when it comes to music, I resist buying digital books.
(Even when most of my friends who had Amazon lists requested them this year.)
For me, having a physical manga volume is one of the main reasons to buy the professionally-translated book in the first place. If I bought a digital version then the only difference between that and the fan version is the quality of the translation.
Sometimes, however, that quality makes a big difference. Fan translations can vary wildly. In some cases the translator opts for a literal word-for-word translation, either because of principals (not all fans agree with a non-literal interpretation), or because their grasp of both languages is less than perfect. But the results can be difficult to read. The meaning of the words is there, but the text hardly flows well in English. Dialog is stilted, concepts get repeated, the deeper meaning of what is being said is obscured. It takes a talented translator to not only translate the meaning of the words but also to make sure it flows well in English and that the concepts being discussed are clear.
I've been comparing chapters of That time I Got Reincarnated as a Slime. The fan translation is by Shurim, who does everything, and eventually stopped translating this after someone complained, and then another translator picked it up. The professional translation is by Stephen Paul for Kodansha Comics. I'm not trying to attack Shurim here, just wanted to point out the difference that buying a professionally-translated manga can provide. This is all from chapter 18, the start of book 4:
The existence that brings disaster to the world, Orc Lord
The skill that is in-born when the Orc Lord appears is a skill that effects all his subordinates: Unique Skill [Starving Ones].
A beast descending upon the world, bringing disaster and chaos. The Orc Lord.
His terrifying innate skill affects all those under his rule. The Unique Skill, "Starved".
Rumuru: Suppress the Orc Lord? That's... asking me to enter this battle? [What is this big sister talking about]
Dryad: That's of course, Rimuru Tempest-Sama
Benimaru: For you to appear out of no where and just start talking, Dryad that's called Layato correct, Why come to this village, There's races that are stronger than the goblins correct
Dryad: You're right. If the ogre village did not fall... I might have went to the ogre's village instead. Mah, even so I couldn't ignore the existence of this master.
Rimuru: Vanquish the Orc Lord? Umm... me? [What is this lady saying?]Fan:
Dryad: That is correct, Lord Rimuru Tempest.
Benimaru: That's quite a demand, coming from someone who just waltzed in out of nowhere, "Treyni the Dryad" or so you call yourself. Why have you come here? There must be other races that would be better suited than the goblins.
Dryad: It is a good question. If your ogre village were still standing now, I might have come to you for help instead. But even if that were the case, I do not think I could ignore the presence of the one who now stands before me.
Rimuru: But whether the Orc Lord has been born is still a question...Pro:
Dryad: We dryads are able to gain many information as long as it's inside the Jura Forest. The Orc Lord? His already born.
Rimuru: Layato Please allow me to reply after a few days.
Rimuru: We had only hypothesized that the Orc Lord was roaming about.Fan:
Dryad: Dryads are always aware of what occurs within the forest. And there is most certainly an Orc Lord here.
Rimuru: I must ask you to wait a bit for my final answer, Treyni...
Shuna: Souei you've went back to our village right?Pro:
Shuna: From your expression... It musts be not found?
Souei: Yes... neither the orc's nor our own kind. There was nothing.
Rimuru: Nothing? What are you guys talking about?
Benimaru: No wonder... I was wondering how they were able to feed 200,000 orcs.
Hakuro: As they do not have any idea in war.
Dryad: Unique Skill [Starved Ones]. A skill that the Orc Lord gains the moment it's born. It allows the orcs to eat anything and everything. This point is similar to your skill [Predator]. Though it's similar to [Predator] but it's success rate is not constant, but if there's many using the skill for consumption at the same time the rate rises.
Shuna: Did you inspect the ruins of our village, Souei?
Souei: I did.
Shuna: And were they empty after all?
Souei: They were. I did not find a single one -- of ours or theirs.
Rimuru: Pardon me, not a single what?
Souei: Dead body.
Benimaru: Ah, I see... I was wondering how they were getting enough food to support an army of 200,000.
Hakuro: Indeed... they have no concept of supply-line logistics.
Dryad: The Unique Skill: Starved. It is a skill that every Orc Lord is born with. It allows him to absorb the properties of any monster he eats. In that aspect, it is similar to your "Predator" skill. Unlike "Predator", there is no guarantee of success in a single attempt, but as his ravenous hunger compels him to devour more and more, the odds will naturally increase.
As you can easily see, the professional translation is likely not quite as literal but feels like natural English, and communicates things clearly. I think the decision that stands out the most to me is having Hakuro say "they have no concept of supply-line logistics". That does not sound like a phrase that anyone in a medieval setting would use... but it communicates what he is trying to say much better than "they do not have any idea in war". Maybe there's a third way to phrase that which doesn't sound as anachronistic, although I have to admit that I probably only focused on it because I was reading the text carefully to note how each translator phrased things. If I had been reading normally it might not have jumped out at me.
The end result is that I can read the fan translation and understand most of the story, but I enjoy reading the professional translation more. It feels more alive, the dialog feels more natural and real. Whether that's worth the cost of a digital version of the comic, I'm not sure... for me, having a physical manga in my hands is still what makes it worth the price to purchase.