I just cancelled one of my Tivo boxes on saturday. I built a silent HTPC and it passed the wife test, so we ditched the tivo. I built it from spare parts, aside from the case that I paid 100 for and the dual tuner card, 125. So out of pocket expense was 225 (it wont be that cheap for most, 2.8 GHz processors arent just laying around most peoples spare parts piles).
My experience:
MCE does everything tivo does, some things better. MCE certainly integrates with my gigabit ethernet much better. Transferring shows from my HTPC to my desktop happens FAST. Not like the lousy usb-ethernet or wireless G "support" tivo has.
Addons are great - just like tivo had started with, I have weather, traffic cams, email, etc - the possibilites are endless. as long as people are coding these programs ill have more to add.
Hard drive upgrades are now very simple. No linux boot cd (I run linux on my desktop so that was not trouble for me), no image copying, nothing. just add a 2nd hard drive (internal or external) and use it.
Hardware upgrades are easy. Its a PC! Just add a HD tuner if you want one. No expensive hardware purchases (after the initial purchase)
Simpler video support - I can plug my dig video cam into the front firewire port on my HTPC, and watch directly from MCE. Memory card reader lets me do this with my dig cam content as well. Also has more supported video types. Stick in a burned CD with Xvid content and it plays. I can transfer movies ive downloaded to it (in a minute or 2 with gigabit ethernet) and it shows up in my list of movies. No conversions, nothing.
NO MONTHLY FEES (lifetime is spendy and your stuck with dated hardware by the time you've saved anything from doing it).
I am an upgrade junkie. I dont like leaving things sitting there for years and years. I want things to get better, and if I can do it on my own with some program updates, new add-ons, etc...I can now do that with no problems.
Thats all I can think of right now, im sure there is more...
Tivo was great for me for a few years, and probably will continue to get better. I still have a tivo in my bedroom (stuck in a contract till 11/06, stupid mail in rebate!

) but will probably spend 200 on a media center extender and ditch that also. Sharing content around my house just got much easier (not quite as cheap as a SA tivo but much cleaner and nicer)
Id say if you can do it, build a HTPC and see for yourself. My harmony remote integrated seamlessly (wife approved) and my tv watching experience just got better (dual tuners alone is enough to change me).
If you have CONSTRUCTIVE comments/questions, id be happy to answer them. I dont want this to sound like a Tivo bashing post, and I dont want the fanboys to jump down my throat either. Its a decision I chose to make, and I couldnt be happier with my choice.