I have and use both a Tivo S3 and HDHR Prime, and they both have their place. Recently my Tivo had a bad power supply and HDD and I questioned whether I needed to repair it or just go forward with HDHR.
After using the Windows 7 with the HDHR for a few weeks it was clear to me that I really wanted my Tivo back. Not that Windows 7 Media Center is awful or anything, but the small differences stacked up and it was clear I preferred the way Tivo works to MC.
I have both as well and am seriously considering leaving Tivo. I have the 3 tuner HD Homerun so I can record 3 shows at once and watch any of them from any TV in the house. The downside is that you must have a PC for each TV.
I have some purposebuilt PCs that are very small and mount behind my TVs. I feel I will save 3 subscription fees. The Windows Media Center remote is not as nice as a Tivo remote, though.