Howdy Biscuit! Welcome to the Cruzbike forum.
Although I have no knee problems, I am also "in the biscuit range" of 300lbs. I've been riding recumbent for about 8 yrs, starting with a trike (Catrike) and then moving to a low-racer (Optima Baron) and then most recently, a Cruzbike Conversion (actually I'm riding phase II, I upgraded by finding a better frame, components, etc.). I haven't lost much weight (body fat ratio has dropped a good bit)...but my cardio results are excellent. Last checkup, I was able to go 'steady state' on the cardiologist's treadmill...at its highest setting (never exceeding my peak HR). My resting heart-rate has dropped to the 50's and my BP is down to 120/70 (though it was never over 140/80). Doc sez I'm "in great shape for a fat man".
So, what can I say about the Conversion? I find it to be very comfortable, fast, and easy to ride. Since all of my previous rides were quite low to the ground, I'm enjoying the view from the conversion...and really feel quite a bit safer, esp. at intersections. I put 45mi on her last Sunday and 25mi today. Having full suspension is another great quality, the previous bikes all rode like buckboards and though they were/are fun, it is so much nicer to have some compliance. The local suburban streets have all gotten "traffic tables" (speed-bumps) and hitting them at anything over walking speed is inviting disaster on the other bikes. The conversion just soaks 'em up! I routinely hit them at 20mph now and don't even bottom out the suspension.
Knees: For some reason (read: it's the designers fault. Hi John!
), the Cruzbike is really comfortable to pedal...much moreso than either my Catrike or Baron. For identical speeds, I use a lower cadence on the conversion. It really feels like I can produce more power (I say 'feels' because I can't really scientifically document it) on the conversion and I am using higher gears to climb than on my other bikes.
Advice:
it is vital to find a bike that will get your final seat height down really low. My frame is a Mongoose 16" and it is just barely suitable for my 32" inseam. Look thru all the conversions posted here in the forums (mine are in there somewhere) and see what is good and bad.
Search out a donor that has a 1-1/8" threadless-compatible headtube. The old 1" does work (with the requisite quill adapter) but I'm much happier since finding that 1-1/8" frame.
Choose a donor bike that already has most of the components you need...at the quality level you desire. Yes, you can always upgrade...but you end up spending more money replacing components. Also, most MTBs have cranksets unsuitable for road use (unless you plan on riding really slowly or climbing trees), so expect to replace either chainrings or the whole thing.
There's a huge amount of info here in the forums. Use it!