I've never actually used one, but from what I've read around the web I really like samyang lenses
(when they started coming out, I already had a nice set of vintage leitz primes; I'm lacking on the wide angle side of things, and I think I'll go samyang when the time comes to fill that gap)
for example, check out these tests: the samyang 85mm f/1.4 is much, much closer to the way more expensive canon 85mm f/1.2 or zeiss 85mm f/1.4 than to the canon 85mm f/1.8, specially in terms of bokeh, which is really important in a 85mm lens
http://www.photozone.de/canon_eos_ff/483-samyang_85_14_5d?start=1
http://www.photozone.de/canon_eos_ff/536-zeiss85f14eosff?start=1
http://www.photozone.de/canon_eos_ff/502-canon_85f12ff?start=1
http://www.photozone.de/canon_eos_ff/419-canon_85_18_5d?start=1
the samyang 8mm is the fisheye I want; for the 14mm I'm torn between the samyang and the tokina 11-16 (which can work as a 16mm on full-frame bodies); for 24mm I had my eyes on the canon 24mm f/2.8, but I'll have to consider this new samyang lens too
but first I have to wait and see what my next body will be...
(oh, and that's where another samyang advantage comes in: being fully manual, I can get a samyang lens for nikon mount and adapt it to canon, sony, m43, etc, whereas the tokina 11-16 or the canon 24mm would have to get sold if I switched to, say, nikon)