It's SP is also bloody hard and the story isn't really worth a damn, but yeah, overall that's a very good choice as well, and another interesting approach to SP campaigns.
I always found SpellForce games fairly lacking: I was looking forward to a good mix of RPG and RTS, and what I mostly got were mediocre RTS bits and mediocre RPG bits that were connected by using the same characters.

While it's marketed as a tower defence, Creeper World is really more of an unusual rts. You might want to investigate it. There's a generous number of 'demos' on Kongregate (which are really pretty much full length games in themselves), with Training Sim being the first one. The full game contains a campaign mode, a number of challenges, a map editor and a random map generator. Here's the link to Training Sim:

http://www.kongregate.com/games/whiteboardwar/creeper-world-training-sim

I found them quite enjoyable. But I like my games to be slow and pacey, so it might not be to everyones tasts. My problem with most RTS's is that they stress me out to much for me to enjoy them. SpellForce did not do so (Except that bloody level in the second one where you have to attack up a fortified bulwark... )

Original War (GOG)
Company of Heroes (S)
Dawn of War 2: Retribution (S)

and maybe some Men of War (Assault Squad) - it has skirmish based campain and five sides

And, maybe take a look at AI WAR: Fleet Command - it's like 20 hrs long skirmish map... well 'galaxy'.

shoo. You'll love it. Promise.

hah, maybe at some point.

The campaign isn't so hard as long as you make full use of unit carry over, research, stripping missions of all they're worth and of course pause. If you use all the resources available to you then it's a lot easier. Also worth noting is that you can steal back money out of your space program as long as you get in there before it's sent. It does mean that you'll take that bit longer, but it's worth it as long as you invest wisely.

Company of Heroes is a great game. The sequel is probably going to be released sometime in the first half of 2013 so it's a good time to start playing the first. I second Kohan 2. But I always try to get people to play the Kohan games so I "might" be a bit biased. ;)

Emperor: Battle for Dune

As a dune fan, I really enjoyed this.

There is also Ground Control 1 and 2. They're true RTT games, no unit production (at least on GC1, not sure about the second). You can choose which kind of units you want to send on a mission (and they get experience and carry over as well), and then good luck with them.

GC1 did get quite hard towards the end, though. I actually cheated through the end of the game... And the whole expansion, which begins just as hard.

And then, World in Conflict is very similar, and from the same developers. Much newer and heavier, though.

You would probably love Settlers 2

Company of Heroes - very similar to DoW in terms of layout, also very good!

Edit: didnt see someone already said that!

Check Battle Realms, it's here on GOG, a great RTS game. Also I recommend Warhammer's RTS (Dawn of War I & II, Dark Crusade, etc.) or the Total War games (turn based campaign/RTS battles).
Maybe you will enjoy Warlords Battlecry saga, an RTS/RPG mix. It lacks of good graphics and sfx, but has a great amount of races and hero classes, and you can play your hero in multiplayer, skirmishes and campaign modes. It's a personal favourite (I own Warlords Battlecry II and III), and it's on GOG too.

EDIT: Also look after 0 A.D., an open source RTS. Looks great ;)

Have to agree with the games already mentioned so far. A few more I have enjoyed in single player skirmish are Empire Earth and American Conquest of the GOG titles. Also Rise of Nations is quite good with a lot of pregame settings to make the game play how you like it.

I have all settlers from 1-7. Played the first two ones on the trusty Amiga many moons ago.