If you're playing Single Player?

LRM Frigates and Cobalt Light Frigates will be all you need. You can just use your capital ships to destroy the worlds. That is why the Marza Dreadnought is one of my favorite TEC ships. Not only is it awesome in ship vs ship, but it's super-powerful in ship vs planet.

If you're playing Multi-Player, there is no perfect mix.

I've been having some interesting results going pure capital ships early game.

By this I mean 0 other ships..no scouts or colony ships. Your first 100 fleet = 50x2 cap ships.

Utility:
Now colony ships are no big deal, just get a colonizing cap ship (it seems plenty of players prefer that anyway), but the lack of scout ships is kind of a pain. If I don't really know a map, I will make 2 scouts to explore the surrounding planets, and just scuttle them when I'm ready to bring out the 2nd cap. Of course, you need scout ships mid to late game to keep track of your opponent's progress, just whip them back out after your first logistics upgrade.

Battles:
Another downside is you don't seem to have quite the kill rate you would with a fleet, but surprisingly enough...I seem to win every fight against the AI fleet with this. My 2 caps will almost always beat a cap+frigates. It's just that caps are so tough that my 2 ships are blasting away while most of their fleet is already gone.

Economy:
getting that 2nd cap out early enough to be useful (1 cap can handle an asteroid, but you need 2 for most planets) can set you back a bit..you have to make it a priority, so your early economy might suffer a bit. HOWEVER, as I said above, it's RARE to loose a ship. I'm literally going entire games (against AI mind you) without loosing a single ship (when a cap gets low, I just retreat it to the nearest repair bay). Think of all the money you are saving not constantly rebuilding your frigate and cruiser cannon fodder.

Furthermore, by focusing only on caps early game, you are free to focus your research on economy stuff (since you don't need to research the various frigate/cruiser types)

Has anybody else tried this? What are your thoughts on it?

Thanks for the input all! (and to answer Astax and Dairuka, yes for the first 100 and a lil beyond that, and yes, single player only for now). I've toyed around a little more, and LRM frigates massed make for some mighty fine fire support (though if the contrails were further highlighted, you'd suspect the LRM skippers graduated from the UN Spacy Academy of Marksmanship ).

While I can't for the life of me remember the name of the colonizing cap ship, I'm really loving its targeting uplink and the bonuses it gives to its fleet. Ion shot is nice too. And I'll have to try the capship only run (though I suspect I'll drive myself into the ground attempting it).

I just use a swarm of cobalts and an Alkan (Colony cap ship). If I'm in a 4p ffa, I like to build the dreadnaught next and use it to bomb the weak opponent's home planet.