I might be the only one in the world that hates PIP, POP, etc. I see the usefulness of it, but I just use the "last channel" feature.
For me, since I am OTA only, the tivo HD's (w/ Lifetime subs) was one of the few options out there for me to have a HD DVR. I like it cause I can modify it and upgrade/repair it myself without much worry. Family loves the netflix now, but even before that, we liked all the features.
The ATSC tuner is good quality, the QAM tuner is funky (when we had lifeline cable we used it, never used cable cards), but it works.
The tivo does have a round about way of checking free space (at least it used to by telling you have many hours you had left for SD/HD). I've never used that feature in any dvr I've had, so I never missed it. The closest DVR I've had that we've liked as much as Tivo was a dish 622. Uverse DVR was functional and that is about the best way to describe it. Cable dvr sucked.
Just like riffjim said though, each dvr out there has it's strong points and weaknesses, I doubt we'll ever get what many consider a perfect DVR.
I got our tivo's from the refurb link on tivo.com, haven't had any issues. I did immediately upgrade the harddrives using the free utilities out there to make tivo HD's from pretty much any HD out there. I love that I can do that.
My wife loves how easy it is to take tivo recording to her ipod too. Plus we can easily burn tv episodes off PBS for the kids.