I use one and I think it helps.
It's only good if you know what times you should be doing I suppose, and it can help 'encourage' you to push a bit harder if you're a bit off - You need to have some sort of self control though I suppose and not just go silly to find 1 second from somewhere - you know what I mean.
I don't use it during a race coz I'm racing the other bikes (and lap times don't matter), but it's useful in qualifying and erm... test days
Of course it also tells you what lap you're on (or how many you've done) so is useful to know how many laps are left (in a race) - if you don't have a pit board etc.