I have an early '90s MIM Strat that I bought in a pawn shop as a teenager, and it is one of the most comfortable players I've ever had. Even now, as my collection has grown to over a dozen guitars, it's still my default choice most of the time when I want to grab something and play.
But for as comfortable it is to play, owning some really high quality guitars has shown the warts on this one over the years. It started with the pickups, which were the typical dual bar magnet jobs that sound shrill and lifeless to me. Those got ripped out and replaced with the SCn pickups with the S1 switching system several years ago. It was a huge improvement, but there was still something lacking.
The biggest problem is that this guitar really lacked sustain unless I just cranked the overdrive and was particularly poor on the low end. Hitting the open E string sounded more like a thud than the crisp low end I wanted. On several occasions I had to check to see if the string was even in full contact with the saddle, because it just had no attack at all. Finally, I bought the full Callaham upgrade kit and installed it about two weeks ago.
Frankly, I'm shocked by the difference. It sounds like hyperbole to say that it is a completely different instrument, but it really is. The low strings now have that piano like clarity and crisp attack. The sustain is there, even when playing clean. The sound is so much more...substantial. I'm not sure how else to describe it. This is truly a high quality product, and I can't envision having a vintage-bridge Strat that I don't upgrade. I just wish he made a replacement for the two-pivot Deluxe bridge, which I guess I'll have to live with for now.