If your TW dvr is sluggish and loud, you'll probably end up loving the Roamio and think it's night and day better. And you can replace the hard drives whenever you want.
Regarding htpc no longer having to pay a monthly fee for the box, think of the Tivo + lifetime in the same way. I myself wouldn't buy a Tivo w/o lifetime. Yes, $900 ($399 + $499 lifetime) is a lot of money but compare that to how much you'll spend on building a nice htpc. And then consider the residual value of the Tivo if you want to sell it 2, 3 or 4 years down the road (which should be worth ~$400+ then). If you factor in the resell value, the costs might actually seem somewhat reasonable. These Roamio's are new but previous Tivo's have been pretty bullet proof for me. If there ever are issues, it's usually with the hard drive (easy to replace), power supply (less of an issue but still easy to replace), and maybe the fan (easy to replace). I'm confident I can keep my Tivos running for many years.
Do you want to deal with an htpc? Some people have no issues with theirs whatsoever. Others have issues (granted people with Tivos can have their own issues too). Here's one relevant slickdeals forum post that might be useful
. I just know that in my household with wife & kids, an htpc wouldn't fly (and I'm a tech guy). Our Tivos just work.