I've been very impressed with Reach's forge mode (much better than Halo 3's) but I've yet to really forge anything.  I spent some time making a symmetrical king of the hill map I named "The Spider" but it isn't really anything to write home about.
 
The root of my problem is that I rarely have a group to play custom games with and when I do, most people seem to want to play gimmicky game modes on specialized maps vs. playing maps designed for standard modes.  I do have a few guys that invite me in to check out their maps every once in a while, but we struggle to get a group greater than 4 or 5 so it's really more just running around and looking at what they've wrought in between a sporadic gun battle.
 
Now, I also struggle with inspiration for a map...  But, so far I haven't really put any time into coming up with an idea for something as I don't want to spend 20+ hours crafting something only to have it sit in my fileshare and never be played.
 
Hmm, that actually brings up yet another thing for me to bitch about.  Six measly slots in your fileshare, if you don't spring for Bungie Pro, just sucks.  I wish they just gave you X space to use no matter how many files you upload.  Stinks that a screenshot and an 18 minute full game film clip both use one of your six slots.  I constantly shuffle what I have in my fileshare, but I like to have a few choice film clips that I leave up for a while and a rotating 2-3 screenshots that generally only stay up for a few days.  If I design a map I'm proud of and want to share, that is going to take up one of my precious six slots.  If I have a tweaked game mode to go along with it, that's a 2nd slot.  Yeah, yeah, call me the wahmbulance... 

i was going to spring for pro but then i realised noone looks at my filesshare, so i didn't bother. ive been struggling for a while now with a few maps but im at a loss for inspiration, so i feel your pain.