Thanks for the comments everyone
I was excited about getting this teleconverter, based mostly on the reviews of it that I've read, but the more I use it the more I like it. Extension tubes are great, but the increased working distance granted by the teleconverter is amazing, and the IQ isn't bad at all, at least when you're using a good lens. I've tried it out with my sigma 70-300, but I'm waiting for a nice sunny day before I render a final verdict on the combination. I'll be keeping an eye out for a good cheap 28mm prime to test it out with, but in the meantime I'm planning on combining it with my extension tubes. Unfortunately, the combination blocks the pop up flash on my camera, so I'll have to wait for a sunny day to test it.