Worth it, but keep some perspective...
I have the MM-1s; my set up is a 24" ACD, Apple BT keyboard, and 2011 MBA. I originally had hoped that the ACD built-in speakers would be sufficient, but there is no comparison. My criteria was size/appearance (I didn't want speakers taking over my desk), sound, ease-of-use, in that order. After listening to a number of speakers, it came down to the MM-1s and Bose Companion 20s, and I went with the MM-1s because they were smaller. For their size, the MM-1s sound great (certainly backing up the reviews), including at low volume, which was really important to me. Very clear, nice depth, and very accurate (if not thumping) bass. They sound very good at louder volumes, too, but I suspect you can get the same quality sound (including at volumes higher than the MM-1s can generate) for much less with other speakers, but you are paying a premium for the name, size and design with the MM-1s. I knew that going in, but I really wanted a small set of speakers that set up as cleanly as possible.
There are three things I would change if I could. First, there is a faint hiss when the speakers are on but are not playing anything. It's not objectionable or very noticeable, but it is certainly there and enough to want me to turn them off when my home office is very quiet and I'm not listening to anything. The second is that these speakers do not shut off automatically. (The Bose Companion 20s do, which almost convinced me to get them instead.) B&W told me that leaving the MM-1s on constantly would shorten their lifespan, so I do turn them off when I'm done with the computer at night. Finally, the right speaker does get quite warm. The remote isn't the best design, but it's serviceable, and a huge remote would have defeated one of my reasons for getting these speakers.
The nice things I didn't appreciate with the MM-1s until I had them was that you can control the volume with the function keys on the Apple BT keyboard. There is no need to fiddle with the MM-1 remote, assuming that the speakers are on. Also, since I'm using a 24" ACD, when I turn the MM-1s off, the built-in speakers automatically come on (and vice-versa), which is nice for those times when I'm working in silence (and turn the MM-1s off) but still want to hear audio when needed. Switching is automatic.
$500 for a pair of speakers that really only work as computer speakers is a heck of a lot. It's hard to justify their price unless the qualities you value are the ones that set the MM-1s apart from virtually every other computer speaker on the market. If all you want is great sound, and size is not really a consideration, you probably could do equally as well or better with other brands. For my needs, the MM-1s are an almost perfect combination.