I've used TwinMOS in 3 machines, 2 x 1GB my MacBook, 512 MB (in addition to the built in 128) my old iBook 800 and 1GB in my girlfriend's iBook (in addition to her built-in 512 MB).
Never had any problems with it (teh only RAM I've ever had to return was a DOA Kingston chip for my Pismo, way, way back). Not sure it it's (dumb) luck or what, but I'm a happy TwonMOS customer, so far...I even have a (small) USB memory stick from them somewhere...
If they're still the cheapest brand next time I'm buying RAM I'll probably go for it again. My regular shop usually have Crucial, Corsair, Kingston and TwinMOS, and it varies a bit which is cheaper.