We recently upgraded our application to SS 2005.
The process was painless. We simply restored a 2000 backup to 2005, upped the compatibility level, and QA'ed the environment.
After the application went live, we ran a profiler trace, and the Performance Tuning Wizard, reviewed and ran in the recommendations, and immediately we saw a VERY significant performance improvement. We've been through these same hoops on 2000 without the same success...
Oh also, we're running the standard, not enterprise edition. Which is fine for our purposes. So the expense is reduced to around $6k per *physical* cpu.
Upgrading to enterprise, may well have bought extra benefits in terms of the performance boost of seamless (as I understand it) re-use of materialized views. At least Microsoft say it does, but it was outside the upgrade budget.
At this stage, we've seen no issue with stability or anything else..