Tamiya decals are notoriously thick. In the 10 years back in the hobby I don't think I've used Tamiya kit decals yet.The amount of aftermarket decals for a 1/48 Mustang must be nothing short of staggering.

I've used the Tamiya kit decals on several occassions-but I used Solvaset decal solution-strong stuff.  But yes, they are thick on the older released kits....I have heard that some of the newer releases have better decals.

As Marc and Bob said typical Tamiya decals are usable but not the best.

Superscale, Aeromaster, Eagle Strike, Kits World, and Lifelike all make great sets of decals for the P-51. Only problem is there's so much out there it may be hard to settle on just one scheme. 

Tamiya decals are awful. I would suggest avoiding them at all cost. Usable? Yes, but even Solvaset isn't going to settle them into the fine recesses of kits like their new 1/32 stuff. Perhaps on the older 1/48 kits with no surface detail beyond panel lines, but even then I demand the best for each build. Get yourself something printed by Cartograph you wont be sorry.

I'll drink to that  which decal to choose ? there are so many I made mine in FLA-ANG