I'd say that Spa and Motegi are the only ones that are worth the investment as they are proper race circuits. However, if you like to blast your way around a huge oval, then Special Stage Route X may be worth your money but otherwise not so much.
Spa is one of the most historic tracks in the world and it looks beautiful in GT5. Unfortunately there is no day/night cycle but it does have weather changeability. Generally, you can't go wrong with Spa if you like circuit driving. You also get the fictional Kart Space track with Spa and it's decent fun and quite atmospheric.
Motegi may not be the most appealing track but it's great seen from a perspective of driving technique, lots of unforgiving tight corners and a perfect lap feels so rewarding. In addition to that it comes with an oval and three variations. The background scenery is sadly not modeled as thoroughly as Spa but it looks decent enough.