Lost my Knight in two of three games today, once on turn 1 and the other on turn 4 (it killed 10 Chaos Marines and a Chaos Terminator in those turns - I had a cunning plan that I would keep it out of range of my opponent's Havocs, at which point it stood on the sidelines in my deployment zone, with a Techmarine doing a terrible job of fixing it and plinking away with a Melta Cannon doing very little damage indeed until it got a chance to charge a squad of Chaos Marines that got too close to my deployment zone, at which point it promptly got melted by a lone Chaos Terminator with Combi-Melta). Lascannons and especially Meltaguns really do a number on them. A random 24pt dude with a Meltagun can get into a non-Ion Shield arc and is a considerable threat to it. 6 hull points isn't an awful lot for something that costs 370+pts and is only AV13/12/12, and when a random AP2 or better weapon can take 4 of them off in a turn? That's scary.