my personal experience after trying them all on a p4 300mhz, 3gb ram, nvidia fx5500 with 128mb vram...
entbloess seemed to be the most feature rich in terms of customizability. i liked how you could customize the backdrop and the fonts and all sorts of other sillyness. but it didn't work with my multiple monitors (i have a three-headed setup), so i had to keep looking. if you're running one monitor, i reccomend entbloess over most of the other ones, at least if your hardware is similar.
i then went to winplosion, which was decent but pretty basic featurewise. i almost bought it, but their weird, anal registration thing was having difficulties with my firewall. i personally get a bit peeved at anyone that's so bent about piracy that they make it difficult for me to purchase their product, so i moved on.
iEX had all kinds of old VB6 dependencies i didn't feel like installing, so you're on your own there.
winGlance was decent, but again, problems with the multiple monitors.
exposer was the only one out of the bunch i can say performed outright poorly on my system. there didn't seem to be any animation whatsoever really. and while it shows the titlebar zoomed up when you hover over the window, it doesn't recognize my window manager, so if you're using windowblinds, ignore this.
in the end i went with topdesk. i think they've probably changed their demo since last year, because i saw all the features that i got with the final version. it worked fine with multiple monitors, had enough speed options that i found a decent setting for my system, etc
it also had a few things i hadn't found in the ones above, namely it has vista's upcoming 3d task switch as a mode, and was one of the few that got the spatial relationship of the windows right, for "true" expose functionality.
my only complaint was that it didn't notice a few non-standard application windows (winamp, trillian's buddy list window, though it did see the individual chat windows).
i noticed that they're trying to work that out, probably for the 1.5.x cycle (1.4.x is largely cleanup)
another thing it doesn't do too hot is active updates for say, a video window. in that department, entbloess seemed to reign supreme. entbloess also seemed to be the biggest RAM-hog however, particularly with those aggressive settings, and i think it peaked at around 80mb of ram with five windows open. comparitively, topdesk took up around 20 to 40mb, depending on how conservative you set the options.
i'd reccomend entbloess if you have only one monitor, like the active previews feature, and have a lot of RAM you don't mind throwing at a task switcher.
i'd reccomend winPlosion if you're looking for a middle-of-the-road application and don't have too much trouble registering it.
i'd reccomend topdesk if having the effect maintain its spatial positioning properly is important, have multiple monitors, or you also want to be able to mimick the upcoming vista task switch functionality.