I would agree with Dan (like I'd have the guts to disagree

!) that the PID won't greatly enhance your shot quality but I would say that if you get good shots now that the PID will enhance the repeatability of getting those good shots. It's not going to do the work for you but it's going to allow you to do what you already do more easily (or more often).
The worth it part is tough. What are your future plans? Obviously there are lots of folks PID'ing their Silvias and lots for whom that's a good place to be. But, if you're thinking of an upgrade any time soon I'd sure do a little more surfing with Silvia and save the bucks for the new machine. OTOH, if you really like what Silvia does and just want to get the temp nailed down so you get those good shots more often then it's hard to beat the shots Silvia can pull repeatably for the price of the PID.
I just moved from my Silvia after 5 years but it was really due the heating element going out and making a great excuse to do something and wanting more convenience for the way I currently make espresso (the ability to steam and brew at the same time became highly desirable).