Not a fan, although the decline version with weights is the best kind. Tnation just had a pretty good article on core training that's worth a read. Normal situps and crunches just don't do a very good job of loading the abs. Abs and calves are primarily slow twitch muscles, so to really hit them well you need high resistance and lower reps, which is the opposite of what most do.
Current PRs at 242, raw w/ wraps- 525, 355, 605, 1485
Roboro tui, perimo vester adversarius
(Build yourself, destroy your enemy)