As someone who just migrated to Tablo from TiVo, I have a couple thoughts.
1) The TiVo interface/environment is outstanding. It's probably the most elegant out there -- they've been doing it longer than anyone, and it shines. That said...
2) The drive runs constantly, and is a failure point. It can also be slow as molasses when the drive is indexing. Replacing it isn't hard, but it's a pain in the rear... which is why there are companies that specialize in selling TiVo drives.
3) Advertising. In recent years, TiVo splashes ads all over the place. On the 'play' indicator bar, in various sections.
4) TiVo does one thing very well: It records and manages TV shows. It does a lot of things really badly -- Netflix, Amazon, all the add-on apps? They run, but if you're using a Roku, Apple TV, Android box? The Tivo is just sad next to a specialized streaming device.
I was loyal to TiVo for a long time. My first box was a Series 2. I have nothing bad to say about them, but Tablo (if not the device, the concept) is where I think the future is going to land, and it has become a better choice for me. The sheer flexibility of the device wins out.