I bought one of these things (AUD$50!!!).

And to be honest, they are not very good.

Some problems are -
1. the mount is very poor. It basically just uses a self tapping screw into plastic. After awhile, the plastic "thread" erodes out - and the whole thing is stuffed. If you do not unclamp the thing very often - ie just put it on the bike and leave it there - then it is probably ok. But this is a definite weak point.

2. It is VERY hard to align properly. When on flat ground, you adjust the thing back and forth on the handlebars so it reads 0. Even though I have the thing clamped fairly well, this seems to vary all over the place on every single ride.

3. Because it uses a bubble, it is affected by accceleration on the bike. You see it as you take off - the bubble moves back slowly (ie the angle gets less). This is not a big problem on hills are you are usually going at a steady speed. But it does affect the reading - especially if you are trying to zero the thing on flat ground as you are riding.

4. It is very hard to read unless you are right on top of the thing - not too good unless you are on a road bike (where you are bent over more).

I have found that the only real times I use it is when I am walking up a hill. I usually then remember to have a look - and also you have more time. I find that I rarely even look at the thing while I am concentrating on doing the climb itself - it is usually too hard to do anything else.

So while in theory it is a good idea, in practice I find that it is fairly useless.