Why some people don't like playing suboptimal characters varies from person to person. With adepts, the problem tends to be that they generally wind up being shitty hyperspecialized versions of street samurai that can do less stuff and are also worse at the stuff they do. This is OK if you don't have anyone else on your team trying to be good at fighting, but it's easy to accidentally make characters where you have an adept who, say, focused heavily on melee combat, and is just worse at it than the street samurai who picked it up as a secondary thing and can also do other stuff.
Of course, power is relative and you can certainly make adepts work if the GM and other players cooperate. In that sense, playing an adept isn't particularly different from playing a goofy or poorly constructed character of a different archetype. Also, not everyone cares and if it doesn't bother you then you don't need to worry about it.
Why they suck:
First, let me be specific. Unaugmented, non-mystic adepts suck. Augmented adepts are OK, so are mystic adepts (unless you use the SR4+FAQ rules for mystic adepts, instead of the SR4A ones, and don't also throw mystic adepts something else like using the best interpretation of Heightened Concentration, but that's a separate discussion). There's two parts, "how do you know they are shitty" and "why did this happen?"
How I know: you cannot build an unaugmented adept who would not be strictly better off implanting some ware, without doing something like building a guy who sucks at everything because he optimized himself into a super-specific niche like being the best in the world at Artisan (Pie Eating), or by cutting 'ware off as a route by being a critter or infected or having Sensitive System. This one's been discussed to death and you can see
http://forums.shadowrun4.com/index.php?topic=4610.45 for a discussion.
Why did this happen: basically, someone at Catalyst keeps on taking on OH SNAP clauses to adept powers to make them suck, and/or overpricing them. EG, multitasking (1 PP for the ability to Observe In Detail as a free action (useful, but sure as hell not worth 1 PP) and the ability to take 2 free actions per pass. Nice niche power, right? OH SNAP, you can't use the second free action in combat. Wait, when else do you track actions?)
Other lollerific ones: Melanin control. Because it would be SO BROKEN for a black adept to be able to disguise himself as being white with his magic powers. You know, like what he could do with a simple goddamn piece of gear.
Distance Strike: honestly, I'm pretty sure this one is just a joke.
The other main cause is that there are some adept powers that are brutally overpriced and really shouldn't be, like Improved Physical Attribute, and the result is that adepts lose out hard to street samurai because they pay a ton more for things like attribute increases.