I've had a desire to play a guisarme-wielding fighter for a while now. Improved Trip is just too fun.
I sent some guisarme-wielding orcs against my players today, but between grease and glitterdust, the orcs never got into the fight. I'll have to try again another time.
The halberd fighters from two sessions back worked quite well, though. Trip attack ... Improved trip ... smash, PC prone. Very nice.
I had an NPC set up for the party this session with a halberd and the whole nine yards: Combat Reflexes, Expertise, Improved Trip, Improved Disarm, Spring Attack, Whirlwind attack, etc. The plan was to spring attack making trip attacks until cornered, then do whirlwind disarm attacks, followed by trip attacks as attacks of opportunity when they went to retrieve their weapons.
But the party got cold feet and decided not to attack. Sometimes the rats can just smell a trap.
Edit: even though Mr. NPC had a halberd, it would be great with a guisarme, too. PC approaches into reach range, draws an AoO. Execute Trip attack, follow up with disarm. PC is now prone, at reach range, with no weapon. Next round: PC ME to retrieve weapon (draws AoO); move equivalent to stand. Fighter executes trip attack againt, follows up with damage. Fun. Even better in 3.5, when you can Whirlwind Attack with a polearm, and take multiple AoOs against the same PC (though you can't use a trip as part of Whirlwind in 3.5).