Pedals for dodgy knees.

Another vote for Speedplay Zeros, infinitely adjustable float and if you buy a pair of aftermarket Keep-On-Kovers (sic) the cleats will never wear out.

Really easy to clip in and out, like spds, but super secure once in.

Best of all, double sided like spds, and you can add 'platformers' if you want to ride to the pub in your ordinary shoes.

Speedplay for me too  on the road bike - enough float, and they come in variable length - keeping hips, knees and feet in the same line (my right knee in particular tends to swing in and out). Also have wedge to cater for shorter left leg. Not a lot of cartilage left on inside of my right knee - mainly from other sports.  Still using spd on my mountain bike.

It partly depends on why your knee is dodgy.  If, like me, it's dodgy because one of your feet isn't level then no particular pedals will help in the long run and you will need cleat wedges.  Have you tried reading any of Steve Hogg's websites?

Shimano pedals for up to 6 degrees of float, very low maintenance, go on forever and parts/servicing super easy to come by.

Speedplays for dual sided engagement, unlimited float, and lots of maintenance (relatively speaking).

I've got some Ultegra pedals at the moment and fancied a change but to be honest I couldn't find anything I really liked. I've an especially dodgy left knee but it doesn't flare up on the bike with my cleats set up properly - enough float in the yellow cleats to keep my knee happy.

Pedals are personal.  For example, I use 105 SPD-SL with blue cleats and have no knee issues.  My brother in law uses Speedplays and, despite constant adjustment of the float and cleat position, has enough knee pain to keep him off the bike for weeks at a time.

OP, I don't think you can get a usefully straight answer to this on a forum. You just need to try them.

Another vote for Speedplay, Zeros in my case. 

You can be fine with Spd-sl of course but if you're getting pain then the every which way adjustability of Speedplay means you will almost certainly be able to resolve it.  The maintenance issue is overplayed but while you can replace every part, those parts are expensive.  Customer service is excellent though, they sent me some shims completely gratis once even though I'd bought the pedals second hand. 

I would buy them again but the cleats are the weakness in the system design, very sensitive to dirt etc. but they last well.

Which is better is going to be down to the individual, the particular issue with their knees, and their riding style, miles etc. I ride ultegra and dura-ace pedals and I have dodgy knees, and I hae no issues with those pedals at all. I find they provide a more stable base (fot can't tip over as much) as with some of the other models I have tried (including an early version of speedplay). I can only vouch that for me, with slightly iffy knees, that the shimano model is not a limiting factor. Big gear work is another story.

Another vote for Speedplay here. I'm using the Zeros and find them excellent for float and by far the easiest road pedal i've used to clip in to. I was using Time iClics, and they were pretty good - personally found them easier to get in and out of than SPD-SLs and about the same as SPDs, but with a more positive action. The reason I originally changed was I really benefitted from pedal spacers, and couldn't use the Times with the spacers I had (they bolt through the back with a hex key)... can't say i've looked back and would choose the Speedplays over them (or anything else i've tried) even if I had the choice.

The Zeros allow you to limit the float, should you wish, although I tend to run them pretty much 'open'. Never had an unexpecte pull-out with them for what it's worth, although that may say more about my power than anything else If you do decide to try Speedplay, and aren't using the new "walkable" versions, i'd recommend KeepOnKovers for the cleats - weigh bugger all, stay on, grip pretty well and stop the cleat surfaces getting knackered (also improve the  clip in slightly IMO).  I get mine re-greased every 1500-2000km or so by the place I got them from (Bike Science in Bristol) who don't charge, which works for me.

Pedals are quite a personal taste but i'd say it's probably worth you giving Speedplays a go - if you don't like them the resale value is pretty high so I doubt you'd lose much, and might gain a lot. Best of luck with the search.

Just FYI about the Shimano cleats:
 

http://www.wiggle.co.uk/shimano-spd-sl-cleats-2/

I have a (reasonably) dodgy knee & went from the yellow to start with & then to the blue without an issue.

Obviously the key is to get them set up to suit but once they are not had a problem.

Ultegra carbon pedals with yellow cleats (6 degrees of float). Job done.

For reference, blue cleats are 2 degrees and red are fixed.

What's your reason for deserting SPDs? I've used them for years on my road bike.

I used to use SPDs but started getting knee problems when I started doing more miles.

Following advice, I switched to Speedplay X2. No knee problems in the last ~20 yrs.

You can get Speedplay X5s within your budget. X2s are stainless steel axles and better (replaceable) cartridge bearings. X5s have ChroMoly axles and cheaper bearings, IIRC.

Since the original X series of pedals, Speedplay have introduced "light-action" and "zero" pedals which are of similar design but have additional features such as adjustable float. I can't comment on whether they're any better.

The only problem with the Speedplays is that the cleats are large, which makes them difficult to walk in. That doesn't bother me. The furthest I walk in them is getting off the bike and going into the cafe. The cleat covers are worth buying, they're about a tenner and stop any dirt getting in to the cleats when off the bike.

I used to commute and then I used Speedplay Frogs. They have the same free float but a much smaller cleat. I used them with Shimano trainer-type shoes which were comfortable enough to wear all day, even though I was on my feet most of the day.

http://www.speedplay.com