I got a chance to ride a 650 Prairie on Thursday 28 June, and all I can say is WOW !!!!
The power is very good, no waiting for the RPM's to come up and get to the powerband. I was doing power wheelies on grass in high range. I got it up over 40 MPH in the small vacant lot next to the dealership. The 4 wheel drive and front differential lock worked great ( I tried it out, and you can barely steer it on hard pack ). The rear end would easily hang out with some throttle in the loose stuff in 2wd. The engine has a great rumble to it at idle.
The bad things that I noticed were: quite a bit of gearbox noise ( could get quieter wiith break in and synthetic lube ), the crappy little cover for the radiator cap, and the engine was a bit louder at idle than I thought it would be ( although not too loud ), it also wanted to 2 wheel in the tighter turns.
I'm a Polaris lover through and through, but... I may have to make a switch.
I had been debating between a Sportsman 500 HO and a Magnum 500, and was leaning towards the Mag for the simplicity and ruggedness. I had been hoping for a 500 HO Magnum, but I guess there's no need to keep hopes up.