How good the army is depends entirely on how good you are as a player.
That said, there are many people who are very vocal about the "top tier" armies and will turn you off getting the army that you really want.
The stories where the 'underdog' armies actually win are few and far between, because only the top tier players can find such combinations that actually work.
For example, let's take Nitros's list in this thread:
http://www.dakkadakka.com/dakkaforum/posts/list/540940.page
He's running basic chaos space marines, Thousand Sons, and Ahriman, along with other units. However, if the internet will tell you anything about chaos, it is that you should stay away from Ahriman and the Thousand Sons at all costs, and that only Noise Marines and Plague Marines are worth taking.
This guy didn't even take the Heldrake(Supposedly the most over-powered unit in the game) and he did well in a competitive environment.
Another few posters talk about how Berzerkers can be used, a close-combat unit in a game where shooting supposedly outmatches close-combat all the time.
My point is this - do not believe everything you hear on the internet about what is good or bad. Don't start collecting an army that the internet said is good, pick one that you like instead. You'll get far more enjoyment out of the hobby this way, that all the losses in the world couldn't take away.
I myself have spent quite a few weeks painting 30+ Chaos Space Marines which aren't meant to be as good as plague marines, and then I'm going to paint a Rhino, which is meant to be a walking coffin on the tabletop.
And I'm enjoying it!