I recently bought a ReplayTV. I did extensive research first.
Started out looking at FormacTV/DV and eyeTV. Arn told me to consider a settop box. So I looked at Tivo (a friend had one so that helped), replayTV, and MythTV.
There are some great websites that have head to head comparisons between replaytv and tivo.
http://www.sgsw.com/misc/faq.html
http://www.egotron.com/ptv/ptvintro.htm
http://replayfaq.leavensfamily.com/
Cnet also has some reviews of tivo and replaytv.
MythTV is an opensource project that can be found at sourceforge.net. No service fee, just build a pc, install linux, and install mythtv. I almost went this route and have still been thinking about doing it just because it sounds like a fun project. MythTV blows Tivo and Replaytv out of the water with features and usability, but (big but) you have to deal with building a pc and installing and linux.
Basically ReplayTV has better quality video (though I doubt you would notice unless you have a large tv), ethernet (makes networking with your mac easy), ability to exchange files with other replaytvs in your home or over the net (basically p2p services), advanced commercial skip features, and a bundle of other recording features tivo doesn't have.
Tivo seems to have a nicer interface, better recording conflict resolution, and a larger user base.
Both will cost about $300 for the hardware (thou there are rebates for both that can bring the cost down to less than half that). Tivo's monthly cost and life time cost are a bit higher.
Both you can hack to add more hard drives/ bigger hard drives.
Frankly I do not think any of the solutions out there are perfect, but all are way better than having nothing at all. I am a mac user, I have high standards, so I can see quite clearly how each could be improved.
my advice?
MythTV if you are not afraid of Linux.
ReplayTV if features, video quality, and access to the video files are important to you.
TiVO if features and quality are less important than a good interface. (good not great).
