It's probably the music I tried it on, but I didn't have a good experience with the software I tested.
First I tried a mixture of Bossa Nova tracks, with no particular correlation between the calculated BPM and the tempo of the music. Then again, there wasn't a very broad range of tempos in the sample I tried.
For a better test, I used a mixture of slow, medium and fast Blues tracks, dividing them into those categories. Neither product, Mixmeister or Abyssmedia, did any better than chance at finding the true BPM.
I suppose it's not surprising - bossa is a fairly complex rhythm, and even slow blues tends to have a lot "happening" most of the time, even when the beat is very slow. I was asking a lot of the software, and I'm not surprised that it didn't do a very good job.
I would imagine that some other types of music would give better results.