Myself and my other friends (all Halo 3 fans and veterans) pick this map for big-team every time it comes up.
The other maps just aren't that great. We've been playing A LOT, practically non-stop this weekend.
We've played every map enough times to have gotten familiar with them. The majority are pretty awful, and there's not that big of a selection. Not just the large maps, but the smaller ones too. Most are not very well-balanced...or fun. You're either on the
"good" side or not, just like a lot of Halo: Reach maps. The only instance I could think of in Halo 3 was High Ground, but at least they evened it out by giving the lower side of the map the sniper and rockets.
Valhalla is by far one of the greatest, large-sized Halo 3 maps. It's a tried and tested map. We play it because it brings back that nostalgia, not because of the mantis. We don't even touch that. We play on foot and let the randoms take the vehicles.