I've used 4: LM, Guerlain, NARS and RMK.
LM: works well, makes foundation last well into the night, katsey i know you said that it melted on your hand before you could even use it, i haven't had this problem, it's my fave atm.
Guerlain do this AMAZING serum called Midnight Star. It makes my foundation *look* perfect, like it's been airbrushed. You can really tell the difference when you use them. It comes in these teeny capsules, they're actually quite expensive in the UK but so worth it! I can't really afford to use them though so I only buy them when I've got something special coming up. There's 10 in a pack but I seem to use them so quickly when I've got them.
NARS primer is supposed to be the UK leader but there's no way! It's rubbish! I notice very little difference when I'm using it.
RMK primer works well and makes my base last longer but it doesn't keep it as 'fresh' looking as the Midnight Star.
If i could afford the midnight star that would definitely be my choice.