I've got the eTrex20 (and so has Rumble I believe). I like it, used it up at Aviemore to navigate for the national this year - lost it in a puddle and found it 10 minutes later, still working so pretty robust. Screen is not bad, probably a bit weak in direct sunlight, but you can adjust the brightness to suit.
Maps - I *think* you can get OS maps if you are willing to spend the £, I just downloaded the Open Street Free maps which are pretty good - straightforward enough and there's a webpage that takes you through it.
There's a 'trip' screen where you can define what data fields you want displayed (current speed, average speed, elevation, etc), I typically leave it on that.
Battery life is really good, invest in a set of 4 AA rechargeables and a charger. The stem mounts are pretty reasonable cost wise too, I bought a couple, now use it on the road bike instead of a speedo.
I think the major difference between the 20 and 30 is the 30 has wireless so it can record data from the Garmin cadence and heart monitors, which is pretty nice if you are into that sort of thing.
If we end up getting out at the weekend, you can have a wee shot and see what you think.