There are several 5D reviews out there. I read three or four before I bought mine. There are plenty of 5D users on G1, too.
Pros: lighter and cheaper than a 1D series camera; cheapest full-frame sensor digital available; ultra-low noise on all ISOs below 800 (and pretty low even at 1600); excellent positive feel in the hand; good build quality.
Cons: have to attach battery grip (seperate purchase) to get vertical grip (although that can be seen as a pro, too!); quite a bit heavier than the 300D; lacks the weather seals of the 1D series; auto-white-balance often inaccurate under incandescent/tungsten light (easily fixed in Photoshop if you use RAW, which you should, on this camera); requires BIG CF cards if you shoot raw - only about 300 images on a 4G card; utterly merciless on lenses (reveals every weakness, particularly vignetting) - requires good glass (but it gets the best out of good glass...).
The 5D + 135mm f2L is a delightful combination for shooting portraits and half-body shots (good for full body, but you need plenty of room). It is a pleasure to use, and gets scary-good results from shallow depth of field (but superb sharpness) at f2 through to deliciously sharp and clear images at f8 to f16.
It's a fine body, and you are unlikely to regret buying it if you can afford it. Just realise that you will have to afford the 5D body + at least one L lens (unless you already have one or more L lenses) - do NOT imagine that you can attach an 18-55mm and be happy
A 5D + 24-70mm f2.8L - nice start. Add the 135mm f2L, and you have what I use for about 75% of my shooting. Add the 100mm f2.8 Macro and it's 85%. Add the 85mm f1.8 and it's 90%. The remainder is scattered across the 200mm f2.8L, 70-200 f2.8L IS, 50mm f1.4, and 17-40mm f4L.
One last tip - the 16-35mm f2.8L is a fine lens, but doesn't collimate the light sufficiently to work well on the 5D - seems to be a digital-only problem. Get the 17-40 f4L instead (and it's half the price!).