I understand your hesitation. I rented the game at first, too.
I’ve now purchased the game, as I feel the lag issues have been largely resolved. You will no longer experience the characteristic rubber-banding and screen-jumping lag that plagued Brink at release.
Additionally, when you play Level 20 Rank 5 games or search for Freeplay ‘My Rank or Higher’ Big Team Games, you will be matched up against a full host of 8 vs. 8 humans. However, bear in mind that latency will still dictate how quickly bullets register and such (ala CoD, Halo, etc.) but when you get in a good match, the lag isn’t even a factor.
I still think they have some tuning to be done before I’d put the Netcode on par with Bungie’s Halo. For instance, I think Host Selection could be further optimized. Likewise, there are, in my opinion, flaws with the way that Host Migration has been implemented (i.e. Objective Progress resets and you start back at your spawn).
Concerning other purchase-hindering objections, I think that the other major bugs have been dealt with. As far as I know, the game-freeze bugs have been addressed, whereas there are still a number of non-game-breaking bugs that will hopefully get resolved by the time the upcoming free DLC is released (e.g. Light characters equipping Heavy weapons, etc.)
To put things in perspective, I couldn’t, in good conscience, recommend the release (incl. Day 1 patch) version of Brink to my friends. Now, I can say that the game is recommendation-worthy.
Hopefully, this review helps you with your purchase decision.
EDIT: I’ll second what Woflnemesis75 and Jess Alon said about the importance of having Xbox Live Friends who play Brink. With that in mind, there are a number of people from these forums who play Brink regularly (including those guys).