We've got a 500 and a couple of 600's and can't recommend them enough for value for money!
Yes they are slightly heavier than Marmot and ME, but have been reliable, comfortable and durable for our uses. Which is general backpacking, bivvying and the TGO Challenge in nippy old May.
They do have an annoying large velcro tab which secures the neck area, and which can get in the hair of girlies (I don't seem to have that problem <sulk>), but with a good neck baffle, they have proved to be worth it over my Marmot Hydrogen, which although lighter, lets out warm air everytime I turn over.
Although we obviously sell Rab and their quality is second to none, if your budget doesn't go that far, the Alpkit bags in my view represent probably the best value for money on the market. Plus the guys there are very customer service orientated, so any issues or questions I'm sure they will do their upmost to answer.