I know a few people who run Crossfire or SLI and they're happy enough with it, but make sure you read this article
and watch this video
Given that the 560Ti is still a popular card, you might consider getting a more powerful card like a GTX670 or an HD7950, and selling the 560Ti to subsidise the purchase. It will be more beneficial to you in the future to have twice the video RAM and more modern hardware.
In addition, you don't have to use another slot, beef up your power supply, account for extra noise and cooling, wait for SLI profiles to become available for new games etc.
I would always choose to sell off older hardware and use that as a budget booster for newer better things, but that's just me. As I said - I know people who are happy with their dual-GPU setups.