Ok, I think everyone else has mentioned all the pros, which I whole heartedly agree with.
What I will do is make people aware of something VERY important. People talk about the blazing speed of this auto-focus system, and to tell you all the absolute truth, it's not all it's cracked up to be. And here's why...
The autofocus system consists of 45 autofocus points. When you are using an L series lens with aperature 2.8 or larger, the 7 center AF points get upgraded to cross-hair sensitivity.
Now, here's the kicker. I have 2 lenses. A 70-200 2.8L, and a 28-105 USM (non L). I was comparing the AF speed with my 300D (original rebel) and found that using the 28-105, the rebel was kicking the MarkIIn's but. I was using the center AF point for both cameras (which is typically the most sensitive). However, the Mark II N was suffering egregious back-focusing, and was searching for years and couldn't find focus. For days I couldn't figure this out. Then I turned the body to take the picture in portrait format, and suddenly it latched on. Then it hit me.
When you don't have an L series lens, all the autofocus points are HORIZONTAL. I have yet to determine whether you need a 2.8, but I would be willing to bet on it. My short range lens is only a 3.5-4.5 and is non L.
This infuriated me, and you need to know this when purchasing this product. I can no longer get by with my $350 carry around lens, and am forced to dish out at least a grand for a 2.8 L series.
This is huge folks. The cross hair vs. horizontal sensitivity is night and day. You should not even bother purchasing this camera if you do not own an L 2.8 Otherwise you just have a crap-load of slow AF points.
And if you think you're going to see noticeable difference in cross hair autofocusing, you WON'T. The motor speed will be determined through the build of the lens. And when you're talking about cross hair sensitivity, there may or may not be a nano-second of difference. I would be interested in seeing the actual manufacturer's specs... but I gaurantee, you will notice ZERO difference. The key is to activate the cross hairs on this monster...
I'm going to my local shop right now to determine if I need an L 2.8, or just an L... but most likely I'll need an L 2.8, as that's what the manual actually mentions.
It's crazy, cause the original digital rebel (300D) does not require such expensive lenses before you can activate cross-hair sensitivity on the center AF point... and I believe this is the case with most Canon bodies.
I will do more research and try to ammend or write a subsequent review.
I wish someone had made me aware of this before I purchased it... it would definitely have made an impact on my choice.
On more thing I will emphasize that people have already mentioned... the weight.
I shrugged this off as a non-issue. After using it for one day, I realized, it is a huge issue. This camera is heavy, and requires both hands. I would recommend a hand strap to avoid droping it as the grip is more rounded, and not as squared off as the original rebel build. I actually find the rebel more ergonomic to hold because of it.
The major benefit that sets this apart from any other camera... 8.5 FPS. When reading that number, you don't understand the impact until you actually have it in your hand and you hold down the shutter button... it's awesome.
Do I give this a thumbs up or thumbs down? Depends on what you need it for.