I've used Singapore and EPGY extensively with my kids and IXL and ALEKS and Khan briefly. I've also used free worksheets from homeschoolmath.net. All at roughly the 3-6 grade levels.
I do not understand the hype about Khan. It's nice that it is free and the lecture format looks cooler than the older software, but the game-i-fication is a disaster with kids with any perfectionist streak and cannot be turned off. It was the first thing we looked at (because it was free) and it got us interested in paying a little for something better. So maybe it has a purpose as an introduction to the space.
I agree with the comments that IXL and ALEKS are fine for exercise drill, but are not comprehensive curricula.
We've settled on a combination of EPGY and Singapore. The open enrollment option works about to about $13/mo. The UI has one or two quirks, but you figure them out and move on. We haven't experienced any disorganization, but then again we have not yet gotten to the courses with human tutors.
Singapore's word and challenge problems are great -- a good introduction to Math Olympiad type problems. The mental math books are really helpful too. The curriculum itself is a bit less comprehensive than EPGY and it requires more hands-on instruction. You'll still need to do a little in-person instruction with EPGY, but it's the most standalone of the options we've tried. The fact that it often explains why your specific answer is wrong is a nice (and, of what we've looked at, unique) touch. So the combination works well -- essentially EPGY for bread and butter, Singapore for enrichment.
