I bought an STI Spartan 9 two years ago. Since then they performed an arithmetic operation and now STI makes a Spartan 4 and 5. Enough math humour, I essentially have the Spartan 5 in 9mm. I've shot the gun for about two years now and my only complaint is that the parkerized finish isn't standing up very well. I've been using a Blade-Tech OWB holster mounted on a paddle. The muzzle end of the slide has worn considerably from drawing the pistol.
I also purchased recently an STI magwell complete with a flat mainspring housing. The new housing replaces the old plastic housing. Overall the quality of the part was acceptable but I spent a great deal of time fitting the mainspring housing. When I first installed the mainspring housing it caused the grip safety to stick in the fire position. With fair bit of judicious polishing I managed to restore the grip safety function but now I'm going to have to reblue the end of the of the mainspring housing.
I much prefer to have these thing fit correctly out of the box but overall I can't say that the magwell with mainspring housing was poor product. It expressly indicated that would need to be fitted and the fitting was a minor irritant at best. Now I'm just awaiting my new grips from VZ and I'll be off to the races.