Hi there, I've had a 450h SEL for about 6 months. As mentioned earlier, economy wise it's not like buying a diesel, but it is WAY more economical than other cars with comparable performance. I used to have an E500, the 450h is easily as quick, both on paper and in reality, and is about 40% more economical I'd say. If I drive it like a nun I can get around 35 mpg, but that's being VERY light footed. And it goes so well that you rarely drive it like a nun.
Downsides? It depends what you're used to. I found the ride significantly harder than the Merc, but not enough to stop me getting it. The biggest negative is the size of the boot, which you'll have spotted and gasped at when you tried yours out. Enough for the weekly family shop, but not tons more. If you want to carry a lot, you'll need roof bars and a box (which my local Lexus dealer has offered to loan me on demand. I've not tried them out yet, but I've no doubt they'll honour it)
One last point. It's really nice to drive something you don't see by the hundred. But that's got a downside too. if your windscreen gets smashed (as mine did last week), windscreen repair people seem incapable of getting a new one anything like as quickly as they can get one for a Merc or BMW or...
I'd still say go for it :)