My own experience was that 5-HTP had a VERY noticeable effect on my sleep for the first week or two, after which point I built up a tolerance to it in that respect at least. I switched to melatonin after the problems I mentioned above, and that had a much less noticeable effect, but the effect didn't taper off at all as it did with 5-HTP...

It depends on the person; there are genetic factors involved with processing of these kinds of things. 5-HTP is a partial prodrug for melatonin and I don't think is generally less useful than melatonin for sleeping.