A lot is said about Combat Mission's incredibly accuracy and attention to historic detail. I'll bet, though, that this doesn't interest everyone...
The good news is, even of you don't know your pazerfausts from your panzershcrecks this is a brilliantly fun wargame.
The key feature is the 'We-Go' turn system, where both players give a minute's worth of others then watch as the battle unfolds, a little at a time. This revolutionary - it captures all the micro-management and deep thinking of a traditional; turn-based game while somehow being more exciting than most real-time games, since you are able to sit back and watch your daring pincer movement rather than be distracted by other things.
The game, I would say, is not even all that complex: once you've got to grips with moving your troops from cover to cover, it all falls into place and the interface seems delightfully simple.
The downside of the game is its graphics - they do matter, as much of the enjoyment of newer games like Company of Heroes is the verisimilitude. However, the graphics are not as bad as they seem in screenshots, and there is a wonderful level of detail on tanks and vehicles.
Whatever kind of wargame fan you are, don't miss this absolute classic!