I have actually ran dual GPU setups and I can say that it is worth it if you know what you are doing. If you look at the prices of the cards I am looking at you will see that current high end cards are way out of my budget ... I like to eat good food you know I am not the type to eat rammen noodles for a month to fund a gaming PC upgrade LOL.
You're preaching to the choir. I know what it's like to run crossfire/sli. I have a system with 2x 290x and many dual gpus setup ups before that. I know the limitations. That's what I'm trying to tell you. It sucks compared to a single card solution. You have more heat, power consumptions, frametime issues, lack of crossfire support, weird glitches, and the list goes on, and on.
Over the past 2 years, games haven't been keeping up with dual gpus. SLI/XF support are nearly always non-existent on launch. It usually takes a good 3-4 weeks before support shows up. Heck, some games aren't even supported yet. That sucks b/c you've had basically bought the 2nd card for nothing. I run into a lot of these problems over the years. The 2x 290x will be my last XF setup. I'm done with them.
Don't get me wrong; when it works, it works very well. The performance is amazing. It just doesn't work as well as anywhere near a single card solution.
However, if you're goal is to save money and this is being treated as a stop gap, go for it. I just feel there are better options without breaking the bank. The r9 290 can be had around $200 and should be as fast as 2x 7850.