Last night the club played a couple of battles using the Sci-Fi version of the Panzer8 rules. Panzer 8 is an extremely streamlined set of rules designed for 6mm figures (works fine with 10mm or 3mm also) that has been adapted for many different settings. Each variant is free, only 2 pages long, and can be found here:
http://panzer8.weebly.com/rules.html
I really enjoyed this ruleset. At only 2 pages, there are inevitably a few areas that are a bit vague that might require houserules but that's to be expected. The basic gameplay mechanics were all quite fast and worked well, and the points system seemed relatively balanced. Notably, the game puts movement after firing, so a bit more forethought is required.
What I liked most about the game was the ability to field an absolutely massive army and still get a battle done in 1-2 hours. I've got a large 10mm sci-fi army made of Mechwarrior figs, but playing Pz8 really got me in the mood to base up the rest of the units I have waiting to be added.
We used our rebased mechwarrior minis and effect was quite impressive. One of our members took the time to report both our games and give a brief review of the rules on our blog here:
http://chicagoskirmish.blogspot.com/2012/09/trying-out-panzer-8-for-quick-play-sci.html
A few pictures to encourage you to visit the full blog posting above.