I run Great Haste slightly differently than it is written in GURPS Magic, but the changes make enough difference to be worth talking about.
As written, Great Haste gives you one level of Altered Time Rate for 10 seconds.
I run it that way, but:
- ATR, as written, does not allow you to Feint normally or otherwise do actions that take advantage of a reaction from a foe. I allow Feints - I feel that a Quick Contest of Skill allowing you to find a glaring gap in defenses or time your quick strike is reasonable, especially as it costs an attack you otherwise could have taken. Plus, this means Great Haste's maneuvers are not different substantially from normal maneuvers.
- I don't allow any quicker thought processes. You don't get unlimited time "out of combat" for this - you're really only getting 2 seconds worth of time in 1 second, so the non-combat usage of ATR is not included in this.
The main reason I do this is because that's how we ran Great Haste that way in 3rd edition, in my 3rd/4th crossover game, and even earlier than that. It's just easier to say "You get two normal maneuvers stacked one after the other" than "except for the following."
Would I change ATR in a game? Probably not - but then again, I generally don't allow ATR on PCs anyway, so it's not a big concern. For a GURPS Magic-based spell system game, I find the minor changes above make the spell that much easier to resolve. In my ideal rules set, the less exceptions, the better.