I used to enter lots of these and picked up the occasional lower placing. They can be useful as an incentive to write, or to finish and polish a particular piece, and if you win a prestigious one, they are certainly career advancing.
I haven't entered any comps for years until recently as I don't write what's in fashion: dystopic futures/urban fantasy/werewolves, vampires etc, but I've just entered Amazon's and am finalising works for the Text for YA children's writing.
Amazon had a lot of fine print and I didn't realise until after I'd uploaded the first story (and not the most commercial) that I was limited to one. Amazon have an interesting system whereby the quality of the pitch, then the excerpt, then the manuscript advances you through each round. A pitch is very different to an synopsis, but I didn't quite wake up to that either until after I had pressed upload. I could have edited if I'd remembered that Feb only had 28 days in it! I'm sure I won't get to round 1 even but writing a 300 word synopsis/pitch was a great exercise and I'll certainly be looking out for next year's comp.