The 550i is a great car, and an awesome engine. For 2008 though, the 5 Series got A LOT of updates, changes and improvements. It would be a shame to buy a 2007 550i, knowing the 2008 is much better. To name just a few changes the 2008 has:
New headlights, awesome new LED taillights, super cool transmission selector (like my 2008 X5), better interior materials, i-Drive improvements and lots of other electronic updates.