Can be quite messy when you have to clean the clogged up nozzle on the Vallejo paints or have to add water or thinner to the bottle because the paint starts to get to thick/ dry out.
Vallejo can be a bit hit and miss- most of the skin tones in the Game Color range I don't like- worst offender being the Game Color Elf Skintone, lumpy stuff only suitable for gribbly minis like zombies/ nurgley creatures, where lumps and warts applied with a paint brush don't necessarily ruin a paintjob.
The Model Air metallics on the other hand are lovely, it's like a whole new world of metallics.
Most of the Model Color paints I've used so far dry slighlty different to what the wet paint looks like, so it takes a bit of time to get used to them.
Also I've found that you have to shake the paints like mad for some colours to get them to mix properly, I've started throwing bits of tabs and steel balls into the bottles to act as aggitators. Nowhere as bad as the Secret Weapon washes. Mixing is far easier with GW/ Coat'dArms pots.
Overall they do make some nice paints well worth trying.