How much better are MFSL disks?

That is broad question so I will answer it this way. As a general matter I have found MFSL disks (assume you mean LPs not CDs) much better sounding than the average retail pressings. That said many titles that were issued by MFSL have since been reissued as 180, and 200 gm reissues or can be found either as Japanese or European imports. There are many previous threads that discuss the quality of various reissues including comparing them to the MFSL pressings.

I own a number of MFSLs, some gold. As my system improves they sound better and better

The sound quality varies from disc to disc.My Hotel california is amazing where as My Crime of the Century is not as good as a Canadian Audiophile pressing I have. You now can compare MFSL with various versions from Natalius and other specialty pressings like DCC and others.

its all about the masters

I stopped getting them. They sound excellent on a cheap/average analog System but they don't move on with a better system.
The original pressings are different (in general), they get better and better with top analog playback chains.
And most MFSL are not true in tone compared to the original.
Resale is good.