You're right. When I got my first R15-300 (brand new!) in July 2007 it was very disappointing. It failed to record shows. It was very buggy. But I'd never had a TiVo (or any DVR) before so I thought it was pretty nice.
Meanwhile, four years later and endless posts by me in the Cutting Edge forums about the R15 and endless software fixes and updates and the box is really very solid and F-A-S-T! It runs rings around my R22. In fact, my displeasure with the R22 (running the same HD software as the HR21) is a major reason why I haven't even considered commiting to a new 2 year agreement and upgrading to HD service.
Maybe the HR24 is better but you can't guarantee you'll get one so I'll keep bumbling along with my SD setup.
Turns out the boxes contained R10s. The last of the TiVos. My mistake. Sorry for bringing it up.