KAG is my favorite RTS. I've been playing it since it came out in 2001 and only recently downloaded the Steam version to connect with friends who had lost their CDs (I have one signed by the devs... I know, I'm a KAG geek).
It has some of the most dynamic AI and gameplay of its day, and games never feel old to me no matter how many times I play them. I really enjoy the company dynamics, the presence of techs, and the way you can create your own units, maps, and AIs to customize the game to your taste (look up KMod if you're interested). In fact, my partner and I have created 7 epic campaigns that we play all the time, each featuring different Kohan and types of challenges. Tinkering with AIs is also great fun (look for The Awakening fan site to download tons).
Anyway, I recommend KAG if you like retro games and want to try something different. Have fun!