Before committing, spend an hour or 2 browsing here
http://www.zumoforums.com/zumo/index.php
There's a lot of user-type info on there which you'll take forever to find out the hard way.
I've got a 550 (first understand the basic spec differences, 500/550 etc). I had read the above forum front to back before getting it, so was aware of the couple of issues there are with them.
1 - the rubber cover on the contact of the bike mount can perish after just a few weeks (they got the rubber wrong). No big deal, phone Garmin and they replace it no questions, just a bit of hassle. I got mine in 3 days from phoning. Garmin's customer service is almost always excellent (one or 2 cases of complaint, but the vast majority are spot-on).
2 - the bike mount contacts can become electrically noisy on the audio channels. Same comment as for the rubber flap, but some folks are reporting good results with using WD40 to clean the contacts rather than any other electrical cleaner.
3 - there have been quite a few cases of the units "freezing". Usually fixed by holding two buttons down, or if that fails remove the unit's own battery, wait a minute and refit. If that doesn't work, put it on charge for a few hours. If that doesn't work, Garmin will replace it.
I don't know if they are any better/worse than others (TomTom etc), and all the problems get reported on the forum so it's a bit pessimistic sometimes, but the vasty majority are very happy with it. I've had the bike mount replaced due to the rubber issue, but otherwise it's been great.
On bluetooth with the Zumo, it seems to work well, though it doesn't support stereo so your mp3 comes over as mono. Most people have no issues with the Zumo and their phones.
I use a wired headset, it's less hassle than you might think, and does give true stereo. Also there's no battery issue like with the Scala. The Interphone system (more expensive) is rated better than the Scala, and they will do bike-to-bike intercoms up to a couple of hundred metres apparently, which the Scala doesn't (the Teamset is for Rider/pillion).
I got my Zumo from
http://www.handtec.co.uk (cheapest at the time) but they are a bit expensive compared to
http://www.blokesstuff.com who get pretty good reviews (though I haven't used them).
If you do get one (and in no way would I want to put you off, mine's great) then get the replacement finger-screw thing that is advertised on the forum (sticky in general forum) made by some dutch folks. It replaces the standard "security" screw which stops the mount from accidentally releasing, but is so fiddly to use you find you either don't use it or you do but can't be bothered to take the Zumo off when paying for petrol etc so get it nicked by a passing scally.
Fitting depends what bike you're putting it on, the forum has a section on installation. I strongly recommend fitting the bike with a "cigarette lighter" type accessory socket, and fitting a suitable fused plug (I got one from Maplins for £2-50) to the bike power lead Garmin supply, suitably shortened (and do away with the fussy in-line fuse it includes). You can then easily remove the whole mount from the bike in seconds and just pull the plug out, so it doesn't get nicked. The power lead fits into the bike mount with a small rater delicate plug/socket which is best fited with the 2 small screws supplied and then left alone.