I like alt coins and have owned a fair few over the last year.
Most of my current value is in Litecoin, but only because it actually has a real cash value away from bitcoin. I also like Peercoin and recently got very interested in Nxtcoin.
However, the trick is to realise that alt coins are the future. Bitcoin is currently doing the work of introducing crypto currency to the masses, but it isn't going to be the coin used by the mainstream. That is going to be an alt coin.
That is far from certain and too bold a statement. It all depends on whether Bitcoin can adapt and incorporate the innovations altcoins will bring. If it can, whether through things built on top of the Bitcoin protocol or through a hard fork if all else fails, then BTC will become the coin used by the mainstream. And with the amount of people invested in Bitcoin and a strong ASIC network backing it, you know it will do everything it can to stay on top of the crypto foodchain. I think it's always good to diversify into a few promising altcoins, but I wouldn't bet most of my money on them right now.