I picked up an IMASAF exhaust of ebay second hand for a fiver when I first got my 1.9 as the back box had a hole in it. Its the best £5 I've ever spent.
It's still going strong now 4 years on and I've had no issues with it what so ever. They're only £50 new on ebay and I would say worth the saving over an original bmw any day. It's quiet, but probably not quite as smooth as the BMW original.
I replaced it with a Remus stainless steel back box for a couple of weeks, but even though the Remus was a "quiet" stainless exhaust, I still couldn't put up with the drone through towns etc with the hood down so I put the IMASAF one back on. I didn't notice any improvement in performance with the Remus on either.
I can't really comment on whether the IMASAF is more free flowing than the BMW original, but as a cheap quiet replacement I don't think you can go wrong.
Others may have different experiences of these cheap back boxes, but mine has been spot on.