It seems to me that Ethereum has a brighter future than the likes of litecoin etc. Quite simply I think there is a lot of upside still to be had in terms of other coins and new coins still to come that use Ether.
If there should be a realisitic new alternative to Ether (more effective or cheaper) thats when i could see the price performance start to flag.
For now though I think we are on the up side.