we have the angelcare one and love it. you need to have a solid base to whatever you are using it in, so if its a moses basket then you will need to get some plywood etc to fit, but thats easy done.
ours has never given a false alarm, that said we have only been using it for 4 weeks, but it defo works as i keep forgetting to turn it off when i lift her out, and it goes off then for sure!
you can have it set to an audiable beep to indicate that she is breathing, but i found that very irritating, and we opted for the light just.. it flashes every other second or so, i love it as when i wake in the night all i have to do is look at it and i know shes fine.. its obviously not a substitute for actually checking her, but it defo provides some extra reassurance.
its worth reading up on the reviews tho. the reason we opted for the angelcare over the tomme tippee is that there are reports that the tommee tippee is affected by wireless broadband, which could be a pain!