As these other guys have said, don't spend a lot of money on the old js skis. You will be a better rider overall if you spend a bit of time riding an old 550 though. Of course just because your money would be better spent getting an old squarnose for around $1,000 or so doesn't mean it's a total waste putting some performance parts on your 550. My old 550 had a high compression head, pipe, prop, water box, sbn44 carb, intake grate, ride plate, pole spring, handlebars, and probably a lot of other things I'm forgetting. That ski was a blast to ride and I still don't feel that a single dime spent on it was a waste. I got as much enjoyment out of it as I did any of my other skis.
The point where it became less enjoyable was after I'd gotten other skis. Once you've started riding something with more power, then the 550's lack of power will start to be obvious to you. Until that point, with a few mods, it will feel like a rocket with handlebars. If you can find a few nice performance parts for the ski cheap on ebay, I'd say go for it. But keep a more modern ski in mind for the future.
But more important than anything else, go out and ride. Enjoy what you have. Any time spent in the tray will go toward making you a better rider and increase your confidence level. The guys telling you to sell your ski aren't trying to insult you or your ski. They are just trying help you avoid spending money where they feel it would be wasted. I don't think any of it's a waste. If you have a cheap 550 and spend a few hundred dollars making it a bit faster. You are still having as much fun as anyone can have for less than a thousand dollars.
You will find that nothing rides quite like an old js hull. They are skinny and light.. Most people who have been riding newer skis a long time find the js hulls to be too squirrly or hard to handle. But spend some time on one and your confidence on a superjet or whatever will be off the charts.