As a Gran Turismo veteran and as a blogger, let me help sway your interest, NateC. By the way, welcome to GTPlanet. I think if you care anything about the history of Gran Turismo and want to know how far we've come since, GT1 through GT4 are definitely worth playing/getting.
* Gran Turismo 1 still sort of resonates with me as the best of all GT titles. This is even if the ratio of Japanese cars to American and British cars is overwhelming, and even if the graphics are outdated compared to today's graphic standards. And then, there's always the appeal of completing race cars by giving the good old Racing Modification.
* Gran Turismo 2- to me, is the worst of GT1-GT4. However, you can't fault or hate on GT2 for cars or tracks. You got a world of cars with you as well as the debut of rally racing. This game was really the first of the GT titles to truly be an "encyclopedia of cars." The action also included faster and more powerful cars than most of GT1's offerings.
* Gran Turismo 3 lacked a lot of the cars that comprised GT2's lineup, but many of the makes themselves returned. Oh, and don't forget GT now looks much prettier with PS2 power and a lot of cars being re-made and re-modeled. Faster cars and longer races are now part of the equation. It also was the first GT to feature weather effects for track (wet course of Special Stage Route 5) as well as the first GT to feature Formula 1 cars.
* And then... there's Gran Turismo 4. When I joined GTPlanet in 2003, I was looking for GT4 news. An extremely complete makeover of Gran Turismo was made in GT4. Many more cars worldwide with the debut of Le Mans Prototypes and "Special Cars" filled the car roster. On the track front, this game actually has the most diverse profile of tracks as far as real-world locations are concerned. It's also the first time you get to race Le Mans (Circuit de la Sarthe) and the Nürburgring Nordschleife. Despite all of this, the most controversial element was B-Spec. People thought this feature allowed the game play itself for you. And of course with GT5 and GT6, a lot of the cars are from GT4. So you get to use these cars in their GT4 state before GT5 and GT6 made them available granted you had the money, and granted you find them in the Used lot in GT5.
So absolutely- pick up GT1 through GT4 for nostalgia's sake and to live out the history of the Gran Turismo series heading up to this point.