We got one and I think it did (sometimes) help the DCs to sleep, but it did have some downsides.
First off, we never used the noise box inside the bear (and if your DC is tiny I think they don't recommend cuddly toys in bed) - we always just had it hanging up by the cot, so the bear was pretty pointless.
Second, the noise only runs for a relatively short time (think it was 5 minutes?) whereas the DCs sometimes took a heck of a lot longer than that to fall asleep, at least when they were little. So often it would stop making the noise just as they were drifting off, and they would wake again. When desperate I was known to hover nearby and clockwatch so I could quickly switch it off and back on just before the 5 mins were up so it would keep going 
.
It is also sound and motion-sensitive, so unless switched off afterwards, if they cried out in their sleep or rolled over and bumped the cot near it, it would start off, which sometimes disturbed me (though it didn't often seem to disturb them).
If I was looking for one now, I think I would probably not go for the Slumber bear again (unless they have improved it since) but look for a more generic white-noise machine with a bit more adjustable settings, so we could get it to run for a longer time (and maybe fade out at the end instead of just stopping dead).
Also do consider Ebay as I wouldn't be surprised if there are a lot of second-hand ones available. Especially if you are just after the noisebox not the bear, so being used matters less!