I use an amalgamated sort of percentage activity system, modified from various RM sources (but drawing mostly from SM:P). Percentage movement isn't a problem for us, since we always play on a map (I've also just cribbed the movement from RMU, though we haven't played with it yet). The round sequence is mostly RMC, with long and short actions. We used the snap/normal/deliberate system briefly, but in sci-fi - especially with player access to Haste - it was allowing FAR too many attacks per round. (Character with twin pistols = 6 attacks per round at no penalties other than for offhand attacks.)
I have now stipulated that a character is only allow a maximum of two combat actions per round (which includes attacks and most spells*), regardless of percentage activity.
The current system has the player with the highest initative (initative is rolled once at the start of combat) declare their first action, and if it is a short action, resolve it immediately and keep going until their next action would take them to 51% activity (101% if Hasted), where upon in moves to the next and so on, and then back to the first character for long actions. This splits the difference between the various RM systems and D&D 3.x's cyclic system, which I think was extremely well-designed (and on into 4E's; while not a 4E fan as purely a system it's mechanically sound - it's what it does is entirely the wrong direction for me).
This still gives Haste a huge advantage, since it means you will likely get both potential attacks in the short action phase, but doesn't make it ridiculous.
Granted, with the old RM2/SM2 phased system, this was less of an issue, but I always found the phased system slow and clunky, especially with a party size of 6-8. Getting stuff done around the table also why I did away with action declarations and hybridised with 3.x somewhat.
*Spells have drastically been reduced in casting time (though I've adapted RMU's SCR system as well for the net game), as otherwise, for instance, EAR spells became laughably bad compared to sci-fi guns.