The biggest advantage if a TiVo based device over the R15 is you have around 100gig of drive you can use. I have about 1.5 terabytes of storage over 4 TiVo's. So the ability to easily expand storage capacity if you choose. Obviously less important with a 100gig base that it was back in the 40gig days, but still important to some.
Then there are other "issues" such as the 100 item limit in the ToDo list, the FF problems, no 30sec skip, no autocorrect on FF, issues with SP guide data to name a few.
Edit: And the R15 as of today, does not allow you to dual buffer programs. ie be able to jump back and forth between tuners keeping the 30 minute buffer on each. Real handy for watching 2 sporting events at the same time. There is rumor of a software upgrade to fix this, but no time frame is given.