I am considering a trip there also - a toss up between that place and Isla Blanca. As Nevada Caster indicated they go out for the big tarpon right offshore generally in the mornings (60# to 200#) and then go into the mangroves for baby tarpon in the afternoon.
Isla Blanca is a bit farther south - I was there 2 years ago. No big tarpon but good numbers of baby tarpon and good shots at permit and some bones (not found in great numbers at Holbox).
Bill Kiene (Kiene's Fly Shop) in Sacramento has been fishing both places for a number of years and has quite a bit of information on both locations. Check his web site www.kiene.com or give him a call.
I really got hooked on going after the permit and my brother and I both hooked slams on our last day but did not land them all. But from what I understand talking to Bill, the tarpon fishing is likely better at Holbox.