Mustangs are as good as anything else. They are a "do anything" cars. There are full of after maket parts to modify them and go as far as you want both money and performance wise. But from stock form, the brakes will be your first issue. Not performance and not handling.
But as Max says, they are heavy and not the most nimble.... but quite fun! I've driven a few and came out with a smile.... OK not the ultimate machine but who cares.... it makes great noise!
My 2 cents...
Ask Carl at Perry Performance for pad and brake fluid upgrades. You don't need a BBK. Only the correct pads for the application. Use dedicated pads for the track and bring spare, it eat pads like breakfast...
With the stock (Koni Orange) suspension, it rolls alot but it's fun if you drive smooth. The main problem is mid-corner bumps. It feels like you are momentarily losing control but you are not. The OE setup is understeer prone, I had a hard time drifting with mine, too much grip!
Have a dedicated wheel set for lapping. The stock tire diameter is quite big and expensive and even on 18", it look like 16" diameter wheels. Use a -1 size for lapping duty. The car will be more responsive, the tire cheaper and more choice than the stock OD. You can easily fit up to 10" wide wheels on all 4 corners. 275/35/18 is a good tire size for lapping (but ulgy for anything else). 19" and 20" rims look awesome for daily driving.
The sound is amazing.
The seats are slippy and not bolstered enough. If you find a Mustang without side airbags, change the seats!
The parts are mostly inexpensive either for performance aftermarket or OEM parts. If you track the car, it will eat trough tire and brake pads fast (compared to a Civic).
I sold mine because I had no time to go lapping with it and I hate driving a fast car slow.