I have been running VISTA /w Cablecards for two years, its been a bumpy road and even though there are still cable company related issues (getting cable companies to give working cards and pair them corectly or getting them to send folks competent enough to diagnose signal issues) the hardware and software actually works pretty well (even my pre-release hardware).
If you have been thinking about getting a Cablecard ready PC they are now available from a number of vendors now: Sony, HP, Velocity Micro, Niveus, and Dell.
If there is interest from folks I can also write a post on diagnosing cable issues, just let me know.
On that note there is a review of the technology on cnet.
[Update 08/07/07] For those of you who are wondering if the Cablecard experience is any better with TIVO, check out the article from Business Week, where it says "It took Comcast (CMCSA ) installers two trips to my house—a total of about four hours' work—and extensive consultations with TiVo technicians to get the unit running properly. This isn't going to cut it. Cable operators and their research arm, CableLabs, must make plug-and-play a reality or the cost will be ruinous." thats consistent with my experiences with MCE, installers are sent out un-prepared, with uninitialized cards, the phone technicians claim they don't support MCE and/or TIVO, etc. once I got on final hardware all the issues I had were with the cable operators but thats unforutnatley to be expected.