I bought the original one over a year ago. I still play it, and I still discover new tactics. The add-on adds a nice touch, not a must-have in my opinion but for the price this sells currently, it's a steal regardless.
It takes a while to get used to the idea of using the overview map as the detail map, and that you can simply zoom in and out wherever your mouse icon is, but after an hour or so you don't know how you ever played any other way. It is very intuitive and feels very natural once you get used to it.
The drawback is that you needed a very powerful machine a year ago. A good machine today should easily and smoothly play this game.
The three original factions play quite similar, with little but important differences in their unit and building set. Some have better shields and can create more durable units, shield their heavy stationary weapons and use a solid "slow steamroller" approach to a battle. Others excel in stealth and use it to flank opponents to catch them with their pants down. Basically, though, all the basic strategies, from "rush 'em" to "one inch at a time" work, and all can be countered easily when you know what your enemy is going to do.
Also, this is one of the few games where maps are HUGE enough to make transporting slow units not only an option but also often a necessity. On huge maps, you cannot simply drive your slow, heavy tanks to battle. You have to find a way to transport them, maybe by airlift, which opens yet another problem, since air transport can only transport few heavy, but many light tanks. What's your strategy? Many small tanks, or few heavy ones? And how do you keep those air transports alive 'til they arrive?
Logistics do become an issue when you can't replenish your losses easily by driving them up there, building a forward base becomes a necessity, and all that counts against your troops limits.
Where do you hit your opponent? Want to take out his heavy gun that keeps pummeling your base? Or will you try to take out his power plants so he can't fire anymore? Hit his production base or better deprive him from the mass necessary to produce? He can't possibly defend both, so where will he be? Can you rely on your radar? Or is he stealthing his units and you send yours into a suicide run?
And do you have a fallback plan when he does the same?
This is maybe one of the most challenging RTS games currently on the market. It's not just "build up and smash him". You can't win with numbers. You can't win by walling and waiting. You usually can't win rushing unless you have a really dumb enemy, his command unit can easily handle your first few units (rushes can still make sense when you can cripple his initial build routine, which can be very essential).
How can you win?
By having the better plan. There is no best plan, so you may have to adapt to what your opponent plays. Be flexible. Adjust and tune your strategy. Never consider yourself already the victor, nothing is easier than blowing up your production or power center and you're flat on your face, scraping together rocks and trees for mass and energy...
This is truely the future of RTS. From interface to unit dependency and interaction, there is nothing in existance that can come close to it.