Ive had a great time in Spec Miatas. My car has been reliable and as economical as you can get-for a race car. I got started with miatas for one simple reason and that was to learn how to drive and carry momentum so at some point I can step back into a faster car-and be faster. Am I going to beat Danny Steyn or Jim Drago-nope, but thats ok, I do this for fun. Those guys do this full time, have huge budgets, years of experience and are probably professional level drivers.

Other reasons I chose miatas:

-Huge fields-no matter what your skill level there is someone to race with. In other less populated series after the first lap or so the field would spread out and I often found myself feeling like I was in a DE
-Excellent wheel to wheel racing - I have learned to bump draft, run in a pack, not be afraid to get close to other cars. Ask a guy in a Lotus Exige or Porsche to do that and he will look at you like you are crazy.
-Cost-For racing its relatively cheap, $800 for a set of tires, used body parts everywhere if you bang things up etc. The Mazdaspeed program gives huge discounts on parts to racers. Id much rather rebuild my miata gearbox for 1k than one for a porsche, lotus etc.
-Tons of Data-Data is a huge way to improve your racecraft. Race a supercharged S2000 and its tough to find someone with a comparable car to compare your data with. Run a spec miata and there are tons of people willing to share their best lap data with you. I had no idea you should be full throttle at Road Atlanta way before the apex of turn one, until I saw the data. Mazdaspeed even offers webinars with professional drivers to help people learn the track before national weekends.
-Track side support-There are tons of companies (in the SE) at the track that can help you with trackside support, transport etc. There are always spare miata parts available if you break during the weekend etc.
-You can race with almost anyone- SCCA, NASA, PBOC. Very rare there is a weekend where I couldn't find a spec miata race somewhere nearby

Is there cheating, I have no doubt. Does Mazda machine their parts as well as Porsche-Nope not a chance. Is the car fast-Nope, Is the car sexy-Hell no. For me its been a fun and economical way to learn in a "sort of spec" series. It has helped me learn to push the limits as a driver. My advice: if you really are in search of car/series-go rent a car and try it out first before you make a decision. Be realistic in your goals, set a budget, see what series are popular/look like fun, and be safe/have fun.