Nifft said:
Momentum is a house rule.
Cheers, -- N
Basic physics in general is a house rule, for that matter.
While yes the idea of something doing x4 moves with a 200' fly speed (800' per round, approx 91 miles per hour) coming to a solid stop on its turn and then dropping like a rock when it stalls may be kind of suspension of disbelief breaking, I find that setting a precedent of playing fast and loose with a heavily rules based system will result in three possible and not mutually exclusive outcomes:
1) Players will expect to be able to get away with almost anything.
2) Balance will go out the window. The DM will then have to start breaking basic rules in favor of the bad guys to keep up.
3) Consistency becomes a pain to maintain.
#1 is not a problem if that is the type of game you want to play (though I'd argue you're not using a rule set best suited for it), but #2 and #3 will become a bigger problem in terms of maintaining a fun game. I find that players will not be happy when they realize turnabout is fair play in an effort to maintain balance, and will squeal like unhappy piggies the first time the DM fudges a not-quite-rules-legal trick for the monsters.
As for the OP though: Time Stop for buffing and battlefield control is pretty darn awesome. At high levels extra actions equals extra win. There are a lot of very strong defensive buffs with short durations that are difficult to get up in combat, but Time Stop gets around that nicely.