When I played this game, I think I had the "Gold" edition, which means I only played the base game and never tried the expansion pack, so I can't comment on that. But the base game is really fantastic. I played it for the first time in 2001, which was already several years after it had been released, so it already looked a bit dated, but man, was it fun. The main campaign is linear but has a decently well-told story and a good variety of missions, from simple firefights against other cars to high-speed chases with loads of cars on your tail, to sneaking into a walled facility undercover.
While in single-player you only get one car, you can upgrade it between missions, changing everything from the engine and armor plating to weapons loadout and "extras" like nitrous boosters or curb feelers. You get this extra equipment by salvaging it from the wreckage of the enemy cars after each mission. This is cool, but it has a few problems, as some other reviewers have noted. One, it doesn't always fit in the story... if I finish a misison where I was basically running for my life, and manage to get away, why am I able to go back and salvage the wrecks? Also, blowing up your enemies doesn't leave the best equipment, for that you need to shoot up their armor a bit and then try to shoot the driver with your handgun. This leaves the enemy car basically intact so you can salvage better items. But, this is both less satisfying than simply blowing them to pieces and harder to accomplish, so the player feels pressured to try to shoot all the drivers instead of blowing stuff up which is more fun.
I never tried multiplayer, but it lets you drive a whole slew of different cars and pick custom loadouts of weapons and such which would probably make for a lot of strategic options. Not sure whether multiplayer is still possible now, but if so it could be fun.
Finally, deserving of its own special mention is the music. The music is simply awesome. The developers got a live band to record some seriously funky '70s style instrumentals that really evoke the nostalgic mood the game is going for. Add to that some quite decent voice acting (especially from the character Taurus) and the style and feel of the cars and the game really feels like a '70s TV show. I really hope that GOG are able to release the soundtrack as bonus material... the tracks that come with the game should be playable already but there was also a separate soundtrack released after the game which featured some tracks that didn't make it into the game.
Anyway, this game is well worth the price on GOG, and I'll definitely be picking it up to give it another go and try the expansion pack out. Maybe other GOG members can get some multiplayer going too.