Just bough a Garmin edge 305 plus HRM belt and cadence sensor for a bargainous price.
I will definitely use the HRM (reason i got the 305 over the 205) but not sure on the cadence sensor.

If i did am aiming to keep my cadence constant throughout a ride by using the gears efficiently?

Haven't tried it on an MTB but use one on my road bike. It's quite interesting to have it but not exactly useful, find the computer/HRM far more important and the Edge is a great bit of kit.

I'd be pretty unconvinced about putting one on an MTB – the sensor really isn't at all robust; I'm on my second one on the road bike as the first one was killed by water getting into it and it's really difficult to stop it from moving around as well. By all means give it a go and report back, but I don't fancy your chances of it lasting TBH.

Aye, cadence isn't that much use on the road, let alone the MTB, don't bother!

It's only really useful on the road to ascertain/maintain your most efficient cadence IMO, in which case you also need a PowerTap or something too. On it's own it's not that handy.

Have used it on my RB and is fine for spot checks, beats counting. Tends to switch it's self off if you stop. Best use is on the turbo where the graph and average in the training center are most meaningful.

On the mtb seems a little pointless unless you are sticking to fire roads, just aiming to keep it high is the best you are going to get, and feed back from the using on the RB gives you a good feel for it anyway.

just aiming to keep it high is the best you are going to get

Not necessarily, see Ullrich vs Armstrong…

If you ride with a lot of tree cover the Garmin may "Auto Pause" from time to time when perhaps it would be better if it didn't. As the cadence sensor also doubles as a backup speed sensor it can be good at giving more accurate speed and distance info when the trees are dense!

I use one off road and its not in a position where it is likely to sustain damage. It shows you how you were riding, ie stomping or spinning. Also seeing the speed you can work out what gear you were in for a given climb and compare it to previous efforts. So I think they are worth using on a mtb.

Where does the sensor mount? Presume something on the cranks??

A magnet on the cranks, and then the sensor mounted onto the chainstay.