I drive honda and would have a hard time switching to something different.
I have an '03 Foreman 450es. Mind you, its not fast, but it will go through the thickest stuff out there.
My friend has taken an interest in the Arctic Cat diesels. You see them on the cover of Auto Trader for $6999 for a 700cc machine, which is A LOT cheaper than what other brands sell 700cc machines for. They seem to run nice, have a LOT of power and its a bonus for him, because he is a farmer (so he can put his coloured fuel in there).
I know a friend that has a 660 grizzly, and have not seen him have problems with it at all. Its got a lot of power can pull wheelies etc and is a nice looking machine.
Another friend has a Suzuki king quad 700. While it looks nice, it seems to have a lot of problems with the radiator getting clogged with mud, etc. when we are out on the trails.
I have seen Can Ams in action on the trails. They are the Ferrari of ATV's, but are priced the same as Ferrari's too (almost literally haha).
Another friend has a 500 Polaris. He has not had any major problems with it. For a 500 it has some raw POWER. But I have heard too often of belts getting wet and slipping. Another friend with a polaris got a little unlucky and has a lemon for a front end. Lots of repair work, lots of $$$.
There are some other things to consider. While it may be nice to have the big, powerful 600, 700, 800 cc machine, they certainly slurp gas a lot more than the mid sized 400-500cc machines.
Another thing to look at is radiator placement. Some of these machines, for how nice the plastic looks, have the radiators in pretty bad places. You may have them damaged fairly easily by trees, rocks, etc.
If I were to recommend an ATV to a new rider or a friend, I would suggest one of two choices. Both happen to be Honda.
The first is the 420 rancher. This machine has a little bit of a sporty look to it, but a lot of utility. It actually runs very quick, topping out at about 95km/h. Being a smaller engine, the ATV has a very light weight. I have 2 friends that ride these machines, and when they go through mud holes, what would be the end of the line for a big bore machine doesn't stop them. The lightness of the machine allows them to bounce and rock the machine out of most ruts. Maintenance is easy and repairs have been few and far between.
The second would be the 500 Foreman. I have one friend that just bought one, and one friend that has a 2004. The style of these machines is very similar even with the 6 year difference in age. Foreman has been the staple of Honda's ATV lineup, and for good reason. Again this is a fast machine, especially the new ones.
Both machines are very good on fuel consumption. It may not seem like an issue with ATV's, but over a weekend/week of riding, the gas bills can pile up. Especially in the bigger machines.
One Honda ATV I would NOT reccomend is the Rincon 680. It isn't a terrible machine, don't get me wrong. The 2 main issues with it are that it only has 3 gears, so in any kind of hilly/rocky/mountainous terrain it does not perform well. The other thing is that the way the gearing works, it performs almost at a 500cc class. Other manufacturer's 700cc machines perform much better. For the difference in price, the 500cc Foreman is a much better decision.
Regardless of the machine you choose, there are/may be a couple of upgrades you want to consider that will improve your performance/versatility drastically.
The first would be tires. Most machines stock tires are functional for general trail riding, but they do not perform well in mud/snow etc. There are 2 types of tires I would recommend. The first would be a Mud Lite. Great tire, doesn't hinder ride quality at all. Most of my friends run these and they tear through the mud. You are probably looking around $600.00 for a set with tax.
The other tire is called an STI Mud Trax. It is similar to a Mud Lite, but the lug pattern is a little different and a bit more aggressive. These are what I run. They are cheaper than Mud Lites- I paid $600.00 for my set, WITH very nice rims. The tread life on them is great and they do not affect the ride quality either. (If anyone would like to know where to get Mud Trax, I can let you know).
The one thing to make sure of with tires is that they fit your machine. Typically machines come with 25" tires. The stock lugs do not add too much height to the tire.
When you are choosing new tires, if you want to go bigger, the most you would want to go is 1 or 2 inches. Anything more than that, and you will need to add spacers and probably a lift kit. Mud Lites and Mud Trax come with either 3/4", 1" or 1 1/2" lugs, which are ON TOP of the tire measurement. Bigger tires also require more power to turn in mud/goop etc., so you may also have to look at putting a clutch kit in to compensate.
The other thing is a winch. Some machines already come with them installed (which is great). Some dont. It is definitely worth the investment.
Do yourself a favour with a winch and buy a WARN. There are winches out there all the time for cheap, but they are cheap for a reason. Most are not sealed against water. Warn are. Check out youtube, there are videos of warn winches being run under water. You will not see that with other winch manufacturers. Warn are fairly expensive, but trust me it is worth the extra money when you are bogged or sunk knowing the thing will still work when you need it.
Thats my 2 cents. Actually, its more like 25 cents lol. Started typing and didn't really stop. Hope some of that info helps.