[Hi Nick,
Executive Summary of a long-winded response
If you don't use iTunes at all and your files are already well-organized, MixMeister appears to be much faster and creates tempos to two decimal places. You still need to create overviews in SSL, though.
If, however, you use iTunes and/or you're going to have to build overviews anyway, you might be better off doing the mapping in SSL to keep things compatible and get all of your processing done in one step.
MixMeister processed files at a rate of (at least) 650 tracks per hour. It is probably higher than that, but I can't be sure because I couldn't get it to work reliably over the network.
SSL processes my files at a rate of about 375 per hour, but is also creating overviews and setting track gain, so take that into account.
Details you can ignore if you want to
I'm currently going through the same thing. I currently have my master library stored on a desktop Macintosh.
I first tried mounting the music drive on my PC and using MixMeister. It chugged along through the night pretty quickly.. I estimate that it got through about 6800 files when I checked on it the next morning. Unfortunately, something had irritated it and it had stopped processing. I could restart the process, but it wouldn't seem to do more than 100 files or so at a stretch. I haven't seen anyone else complain about this issue, though, so I assume it's a problem with my files or something else about my particular setup.
The real deal killer for me, though: MixMeister created really nice tempos down to two decimal places. Unfortunately, iTunes on the Mac choked on them and displayed any MM created tempo as a very large number.. like 64,203. These bpms did appear correctly in SSL, but SSL doesn't display anything to the right of the decimal.
So I decided to try creating tempos in SSL. It seems to get confused a little more often about halves/doubles (e.g., a 144 tempo is listed as 72) but that may be an unfair assessment as SSL has now processed almost 25,000 files - I've had more of a chance to notice the mistakes.
The nice thing is iTunes happily accepts the tempos created by SSL and displays them correctly. Additionally, I've created overviews and set track gain in one step.
Assuming that rate of 375/hour, my library of ~25.5k songs should take approximately 68 hours of processing time on my Dual 2.0Ghz G5 PowerMac. I'd be curious to see if it's any faster on my Core2Duo MacBook Pro.
Assuming a rate of at least 650 tracks per hour, MixMeister would take around 39-40 hours to set all tempos. This is on a 3.0Ghz Pentium 4 processing files on a network volume. Keep in mine that I still wouldn't have the overviews or track gains set and would need to factor that in to the final time needed to get all files completely processed.
Hope this info helps you (and others) make a decision. My preference is to let SSL handle it all. I'm then going to play with the library for a couple of weeks to catch as many incorrect tempos as I can before forcing iTunes to re-read the files and display all tempos.
Cheers and good luck!