Just bought a new 2004 Yamaha 660 Grizzly on Friday. Great bike, just finished the 20 hour break-in period. Already had a Yamaha 400 Kodiak and an old Honda Fourtrax 250. The IRS on the Grizzly creates a whole different ride from my Kodiak. Power and handling to burn. The factory Dunlop tires are better than the factory tires that came with the Kodiak back in 2001, but after the warranty period is up, I'm going to be putting a set of Mud-Bugs or Tri-Claws on her. Side by side, the Grizzly isn't that much bigger than my Kodiak. Only complaint is lack of color choice. Up here in Canada, the 2004 model is only available in red, blue and green. Would've preferred to get Black or Silver Tip configuration, but isn't available.
Can't comment on how dependable the Grizzly is yet, but haven't had any problems with the Kodiak yet. Have used it hauling wood, all terrain/weather conditions. Used it to cross rivers, to retrieve three moose. Worked like a charm. Only complaint is the rear brakes. Have to be careful installing and adjusting rear brake pads, otherwise they'll wear.
As for the old Honda Fourtrax 250. You can't kill it. Bought it new in April of 1986 and you can still grab the kick start with your hand and kick her over. Have used, abused and carried way too much cargo with her and she came through every time. Don't use her much anymore, but haven't had the heart to sell. It'll probably still be working when all these new-fangled LCD displayed super-ATVs are sitting in the dealer's garage waiting to have their electronics looked at.
My pic right now would be the Grizzly, but second would have to be either the 700 Polaris Sportsman and/or 500 H.O. Sportsman. I have a lot of buddies with both models and they regularly use them hauling firewood in winter and moose hunting in the Fall. No complaints and overall good reliability. X-man