There was a very good thread discussing these a while back, but with the search engine being a bit 'clunky' at the moment, it could me some time to find it, but if I do, I will post a link to it.
Support Weapons can be useful, but the main problem with them is, as Seer13 points out, that they are static, which means they are only really going to work in Eldar armies which have a very strong fire base, and are broadly looking to Castle, or play a gun line with perhaps some kind of reserve Deep Strike or outflanking force to catch the opposition off balance. Both of these strategies, however, rely on your opponent coming to you, and this is where the problems start, since if you end up facing Imperial Guard or Tau, they will just stay in their deployment zone and out shoot you, which is why I don't think Support Weapons, in general, work that well in army lists designed to take on all comers.
Shadow Weavers are not too bad, providing you attach them to a well defended fire base, since they have a long range, so you can make them work with more aggressive Eldar lists, providing you ensure that you don't end up fielding a polarised list.
D Cannons, however, have far too short a range for that option to be viable, so they would only work well if you know your opponent will come towards you.
As for Vibrocannons, they might shake the odd vehicle about a bit, but otherwise I don't feel that they are especially useful, so unless you really do want to play a gun line style Eldar army, I don't recommend making room for these.
I hope that helps.