Yep - Optus .681 is great!
I'm on At&T as well, and the Optus 188.8.131.521 is great! Very stable, improved batterry performance than AT&T's latest 4.6 version of the OS, and a better UI. I strongly recommend to anyone and everyone to upgrade to this version if you are on AT&T.
EDIT: Just noticed the latest post, and I can say that I too have been a long time AT&T user, and this too is my first Blackberry. I was very hesitant about trying any "un-official" OS's, but after reading all of the glowing comments about Optus .681 I took the plunge. It was like having a new phone again! I should point out that where I am not a fan-boy of AT&T's by a long shot, I have come to appreciate the level of testing that they do on an OS prior to releasing it as official - as evidenced by other carriers who seem more than willing to release an OS prior to it being fully tested. I'm not bad mouthing Verizon at all, but I don't believe that they have any better tech support for Blackberries than AT&T. One final thought is that the 9000 has been out for 20 months or so, and is definitely at the end of its life cycle. It may be too much to ask of a carrier to keep current on official new OS releases when they are coming out with newer and better products - 9700 and soon to be released 9800. Take the plunge with the non-official version - the water's fine!