MoO1 was a blast - a game which swept me off my feet with it's complex gameplay, large and fairly diverse techtree and truly distinctive races which added to replayability along with randomly generated space.
MoO2 was a shock at the first moment - borrowing a number of features from Master of Magic or, if you like, from civilization (though less so). After a bit of grumbling, I gave the game a chance and I wasn't dissapointed. Frankly, I haven't yet found better space 4X strategy, each I've tried (spaceships unlimited, galactic civilizations...) just made me eventually yearn for this classic and return to it, since they were, in some way, lacking in comparison.
Among things that make the game incredibly enjoyable are the techtree and the dilema which tech to chose (since you can often only develop one of two or three through research - maybe not so realistic, but heck, it's FUN), construction of custom ships (and the equipment really matters, not the usual +1, +2, +3, ... blaster), several options of winning (diplomacy, conquest, beating the Antareans) and there are more. For the price, it's absolutely unmissable - and it would still be for four times as much.