They aren't terribly good, no. The one advantage they have is that they move early and, what with both high movement and long range, they will at least get a chance to do some damage before they die. The problem is that they don't do very much damage, and they can't take damage very well. Generally not worth it.
However, if you keep getting attacked by people with armies of 8 artillery, that stay back and fire at you from across the map, they are probably the most effective defense against that for their age.
Generallyk, what I do, and I am not at all sure this is the best strategy, is to quickly build 2 or 3 barracks each of the first troops of an age (usually short range and light) and then when I get to the end of the age build 3 barracks of the artillery (long range) troops. Then when I get to the next age I deleted the old short range and light and build new from the new age. That worked for me. Not saying it's the best strategy though.
^^what he said is pretty well rounded unit strategy. Archers and light units have taken a buff though since my first time through, while horse and heavy have been strengthened. Still I have built very very few horse or heavy melee barracks on any world during any age, I just never was a huge fan. They both have pretty severe limitations and the barracks usually take up quite a bit more room.
Realistically, after archers and some other units were buffed and AI was improved, Early Middle Ages Units (EMA) kinda stink in general. Actually, no, they always kind of sucked, lol. There will be a significant and noticeable difference in unit strength once you get to High Middle Ages (HMA). So basically, if you're going to wait till you're at the end of EMA tech, you might as well wait for the HMA tech.
They're ok, but yeah, the early middle age units were pretty crappy. I think I stuck with the iron age archers and tried to get through EMA as quickly as I could. EMA is the only age which has that crappy units.
Yeah EMA units were just that -- aka the Dark Ages, after the fall of Rome, you know, nothing was very progressed. Mercs and Armored Infantry are quite useful, I know Catapults do more damage than Ballistas.