The power graph between the 370 ISL, 400 ISL, and 425 ISL is very similar. Basically it's nice and fat until it get's up to the higher revs, then the extra power comes in. The only time that you'll see those extra 25 ponies is when you are climbing a real steep grade and the engine is wound up in excess of 1900 RPM. For 95% of your driving it'll feel the same as the 400 ISL.
The new engines aren't out there that much yet because most people in the market jumped the gun and bought one of the earlier engines to eliminate any hassles with ULSD. Still, they are starting to filter into owners' hands now.
An RVer recently bought a new 2008 Allegro Bus with the 425 ISL on a Freightliner chassis. The Bus is available with your choice of Freightliner, Spartan, or Tiffin's own Powerglide chassis. Previous Freightliner XC units with the 400 ISL and side radiator had a marginal cooling system and had a tendancy to run hot and overheat on steep grades. Freightliner has changed the cooling system on the new ULSD spec chassis (I believe they are even using two fans now) and the owner reported much cooler temperatures compared to the older XC chassis and was happy with the engine for the first 1,400 miles.
However, once he hooked up his toad he melted three Remco lube pumps and there were various other signs of heat damage on the toad. The exhaust from the DPF gets much hotter than the older engines and can reach temps of 1,200 degrees inside the DPF before being exhausted. Unlike Spartan and Powerglide, who direct their tailpipes out the side, Freightliner has chosen to direct their tailpipe straight out the back. This heat is basically melting the toad like a giant flamethrower. Freightliner has stated that they can't go out the side because the frame is now in the way so now they have a problem on their hands.
So, this is something to be aware of if you are looking at a new 425 ISL on a Freightliner chassis. Apparently the engines are running fine but the direction of the exhaust is a problem for anything behind the coach. I'm sure they're going to have to come up with something to remedy this. Exhaust stacks maybe?