I've had pretty good luck over-all with picking chests and coffers. Only one or two Mimics and I've been weakened more by Chests than coffers. I support the theory that being over-leveled and using Tools on chests will likely result in being weakened (level 64 picking a Delkfutt chest).
I mainly carry Thief's Tools. They're cheap and readily available. I believe there's a vendor in each nation that sells them when the nation is in first but only sells them at night (or when some other situation occurs). On most chests, you'll break even if you get gil, on most coffers you'll profit at 4k/tool.
I also always carry a Key Ring belt with a few key charges left on it for the tough ones.
The other day I was in Newton on my way to the solo ENM60, so I popped over and found a coffer. Failed once, got the map on the second tool. Went hunting later on in Ru'Aun Garden and found TWO coffers, popped the first for the map and got weakened on the second with a TTool. Then, while searching for the Curtana ???, I found the coffer in Ve'lugannon Palace. I blew through my last 4 Tools on it and ended up going back to Bastok to get my Key ring belt out of storage. Popped it in Bastok and when I failed on with the Skeleton key upon returning to Ve'[TAB], I waited another 12 minutes on my last key. Used the belt and... popped the coffer!
I have (what I believe) to be all maps for Sky at level 64, never having touched a coffer key party, one week after getting Sky (not to mention SSG, Temple of Uglypeople, Newton Movapolis and a few other coffer maps) and it feels darn good to be a Thief. I won't even go into how pleasant my AF experience was after spending countless hours in Beadeaux coffer parties on Monk!
As far as stats not affecting success in picking coffers, I'm forced to ask for the source of this information. Before that nail-biting Ve'lugannon coffer, I popped a squid sushi and maxed out my ACC with SH and Life belt and I failed. I then put on my cotehardie, RK Belt+1 and AF hands and legs (basically my SATA gear) and after 5 fails without these stats, popped it. Thf relies a lot on luck and a lot of it is in your head (the reason I carry my four-leaf mandragora bud with me) so do what makes you feel best about it and that way, you'll have no regrets or "shoulda-dones".
And as far as denying other people their AF and quests.. I agree. On my way for LB3 I came across a coffer in Monastic Cavern with a Mithra Thf who told me "Go ahead and pick it, I'm out of tools". So instead of getting my 8k I gave her a few tools and watched as she cracked it open. I gotta say that that felt better than cracking the coffer myself would have.
Last night I was in a coffer key party for a Pld friend in Castle Zvahl Baileys when we came across a coffer. We had been killing for an hour with no key and I asked if I could pop it, to which they agreed. I got a Mimic which we killed with ease who gave us our key. I'm curious if you're more likely to get a Mimic when there's someone in the party with an AF quest active? I can't honestly recall having gotten a Mimic without having an AF quest active or someone in my pt with one active. It would almost make sense since the Mimics always drop a key (and we found the coffer 20 minutes later).
Edited, Jun 14th 2006 at 9:52am EDT by MobiusRenoire