I have the Comcast/Tivo DVR and it, in a word, sucks. I actually think that the straight Comcast DVR is better and that isn't saying much. It's not even remotely close to an actual Tivo.
It's like if you recruited Usain Bolt to run a race for your team but just prior to the start made him eat 50 hot dogs, blindfolded him and tied both his legs together. Sure, he'd still mostly look like the fastest guy in the world but he'd probably not go where you wanted, wouldn't be fast and stands a good chance of puking.