Chris,
Personally I use the RAC "Flying Horse" CD ROM version. Since most web-based callbooks are updated on a daily basis, interfacing to a logging (or contesting) program is the only reason to own a CD ROM callbook. I use the Logger program (
http://www.kc4elo.com/) and it works quite
nicely.
The RAC also provides some other ancillary programs on their CD in addition to the callbook. I have looked at them for curiosity's sake, but honestly, I have never used any of them.
QRZ is the clear winner in terms of web-based callbooks. Again, they all have the same information, but their on-screen format is the most "user friendly". Along with that, I think most would agree that probably the most frequently heard thing on the air for both stateside and DX stations relative to sending a QSL card is something like, "address OK on qrz.com". I hear this on CW, all of the digital modes and on phone. Sounds like a pretty good endorsement to me!
73,
de Scott NØIU