I do have 5.1 and I use it for listening music. If that's necessary is another question
Most bands still don't mix their music in 5.1, and I don't think that I have come across a power metal release that was mixed to 5.1 (However, I wasn't actually following that scene lately). If you listen to a regular cd on a 5.1 system you will get either normal stereo (with the center+rear speakers muted) or "double stereo" (don't know if there is a term for that), meaning that you get the normal stereo mix plus the same thing in the rear speakers. It makes the sound bigger, but you don't get any extras or other effects, and I wouldn't consider it a big improvement to 2.1. In fact I would probably prefer a well set-up stereo. Also, you would get the same result if you plug 2 speakers in each stereo output. I do that on my stereo with one speaker in each corner so that the music is equally loud (and stereo

) in the entire room.
Bands that do mix their releases in 5.1. release them on DVD-A, since it's technically impossible to get a 5.1 mix from a regular CD (it has only two channels). Same goes for mp3, if you prefer to download your music. This can be quite nice, as you really get 5 different sounds from all five speakers and music can sound really different this way. Many Bands use it to pan effects (delays and such) so that the repeats come from different directions, or have different background sounds (maybe strings in the front and a synth pad or a hammond in the back). Maybe check if your favourite bands have done any releases before you decide. Notice however, that Audio DVDs can't be played on a regular cd-player, i.e. on your stereo (probably) or in your car.
The last thing are the Video DVD's, mostly Live DVDs. Most bands mix them in 5.1 and you'll get a different sound than with stereo. If it's worth it is a matter of taste, and the music you listen to. When I watch the BG and Sonata Arctica DVD, for example, I don't notice so much difference except for a "bigger" sound, when I watch the Pink Floyd Pulse DVD I notice a huge difference. Imagine "Us And Them", with every echo coming from a different direction. It's great
So if you're a big fan of Live DVDs and such it will probably be worth it. Might be a good idea to stop by a friend with a 5.1 System first, if there is a big difference or not is always depending on the mix, some bands hardly use surround effects while others do extensively .
I hope that helped!
edit: Just shopping amazon for some music to get. I was looking forward to get Steven Wilson's "Grace For Drowning" 5.1 mix, but apparently it's only released as a stereo cd or in 5.1 on blu-ray, which is of little use for me as I don't have a blu-ray player. Just to show that the DVD-A seems to be outdated already, and the every-day use for blu-ray audios seem to be even smaller, since I don't know many people who have a blu-ray player in their stereo setup (or even pc).