It's true that a lot of people are against the whole eggcrate foam thing, but I'll be honest - I've used it in a pinch with surprisingly good results. Look, any type of surface with absorptive properties is going to be better than 4 parallel, untreated, hard surfaces.
That being said, I wouldn't invest in bed foam from your local Walmart for a project like this... If you've got it lying around and you need some quick treatment, the eggcrates are probably going to be better than nothing.
The reason why some people are dead-set against the idea of eggcrates is because of the inconsistent noise reduction coefficients across the frequency spectrum. Because of this, your recordings could potentially end up with problem frequencies (too much of some, not enough of others). But ultimately, if sounds good to you, awesome!
You can get actual acoustic treatment foam for a room that size for very little money, so it might be worth it to do a little Googling. It doesn't have to be Auralex. If you look at the NRC's on the cheap stuff, some compare favorably to the expensive stuff.
To test if you have some problem frequencies in the room, use a good studio monitor and a fairly flat mic. Put the monitor in front of the mic, and pump in some white and/or pink noise. Use a spectrum analyzer and compare the original noise signal to the one that is coming back from your mic. If they're pretty close, looks like your room is set. If not, figure out what the problem frequencies are and Google how to fix them. More than likely, the problems are going to be in the low-end, and adding a bass trap might help.
Sometimes it is also helpful to play some music through the monitor and compare the analyzer results as a "real world" test.
Sorry, I didn't want this post to be this long, but with lessons learned when I built my studio, I figured I'd share.
Rick