K2 Auto series... possibly the best out there.
I'm definitely a gear whore and have gotten my hands on alot of bindings these past few seasons. Recently I picked up a set of 2010 K2 autos, and while I haven't ridden them yet, I've mounted and strapped them on and am thoroughly impressed with their quality. Off the top of my head, I have experience with 2010 cartels (EST and regular), 2010 exiles, 08 rome 390's and 2009 390's, 2010 ride deltas, 09 union forces.
Aside from hiback stiffness (which can somewhat be adjusted with the forward lean) For me bindings are mostly about comfort, practicality and ease of use.
Here's my PERSONAL breakdown on the 2010 K2 autos versus others I've tried.
Weight: EST bindings excluded, these are the lightest regular mounting system bindings I've held. The burton cartels are on the lighter side as well, while the rome bindings are pretty damn heavy. Practically speaking though binding weight is something I only notice riding the lift and sometimes stomping landings.
Ratchets: I find ratchets with protruding tabs for unstrapping the easiest (like the Rides, Romes, and K2's) and k2's is the smoothest. Romes are very nice and Rides are OK... they sometimes stick. Burtons and Unions are a little more difficult to unstrap without the tabs, and Unions seem the worst of the bunch.
Straps: 390's, delta's and autos are ultra comfy, Cartel's hurt my feet after a while, exiles were better than the cartels. Unions were not bad. Rides toe strap is great, while all others were pretty good. Unions toe strap sucks for me. K2's auto system makes sense.. the only thing I'm worried about is durability, however they've already gone through a few generations and I haven't found any problems online yet (cross my fingers).
Cushion: Delta's, cartel's, autos have tons of padding and k2's is canted. Burton exiles is above average, Romes is average, and Unions padding sucks... i don't get why they did it the way they did.
Hibacks: cartels the stiffest and most agressive followed by the K2 autos then deltas... unions feel thin and flimsy. I find generally simple hiback adjusters the most reliable. The one found on the higher end Burton's that are flush with the hiback are a nightmare, and I've had the new rome adjusters loosen up on my many times. I ended up moving the older 08 390 system to my newer ones and all problems disappeared. I much prefer that style and the ride delta and burton exile type. K2 has done something different altogether and integrated the adjustment into inside heelcup area. It makes sense, but takes longer to set up. I'm not one to constantly adjust my hiback once its set, so I'm really liking this idea. Reduces weight and seems reliable.
Adjustability: Rome is hands down the best. Everything else is good, except unions suck.
Baseplates: I generally find stiffness can be adjusted. Little things I notice though are ride and rome provide under baseplate padding that seems to help with dampening and not marking up the boards. Burton baseplates seem to really dig into boards. I actually bent a Ride aluminum baseplate before when I triple-roll bailed going full speed. Having a size 11 foot puts me on the verge of sizing with alot of companies with Large and XL, a little bit.. IIRC, rome's go from size 9.5 - 14, Burton's large 10 and up, Ride XL is 11-13, K2 is 11-15, and union is 10-14.
Again I haven't actually ridden the K2 Autos yet, but so far the design and overall quality seem very impressive. If you can't tell already, I'm a big fan of Rome 390s and Ride Deltas, while Union is at the bottom of my list.