The quality of HK planes worries me (they're such low cost) but I know plenty who have flown them and say they're pretty good, but the instructions can be wrong (CofG etc.), so they often favor a more experienced flyer.
As someone else pointed out, HK doesnt make the planes, even the one's they brand themselves, like the durafly range which is (or rather, was) actually produced by FMS, hardly known for shoddy planes. HK are generally fantastic value for money, but they do sometimes skimp, in particular on the quality of the servo's. Not with all models, some come with decent servo's, some dont. Now you can get lucky with cheap/bad servo's (and unlucky with expensive ones) but it may sometimes be a good idea to replace the critical ones. But even then you end up with a very compelling price for what you get.
As for manual; somewhat of a mixed bag. On some models the manual is surprisingly exhaustive and quite excellent, on others its 2 sheets of A4 in chinglese, but even that is generally enough. Ive not encountered serious flaws with their noted cg
As for other brands; with multiplex you get their fabulous elapor foam, and the rest is usually top notch as well. You do pay for that however, and the difference isnt worth the price premium to me in most cases. YMMV. Similar things apply to FMS themselves, although the price is generally more reasonable and I cant say anything bad about the FMS planes I own (1400mm zero and fw190, and highspeed 950mm P47). But I only got them because hobbyking didnt carry something similar, otherwise, id probably have bought them there.