This is an absolute classic when it comes to real-time strategy. It's like playing Civilisations without the turn-based concept. The graphics are really really good, especially for the time when this game came out.
The gameplay is quite good, very simple to catch on, although you do need to be good at multi-tasking if you're going to remember to check on everything! Very very high replay value - every random map really has its own challenges.
If you like strategy, you really should try this one! Or better, get AoE II!