We used Clear Clearomizer 510 for months. Despite some minor flaws, it's good value for money and we can recommend it to anyone using eGo style electronic cigarettes, especially for trying out new e-liquids. The drawback is that you can't really find it so easily.
UPDATE 2015: this was written back in 2012 and today it's hard to find this clearomizer anywhere, except somewhere at the back of my drawer:). So, if you want to purchase a small, standard clearomizer that doesn't spend too much e-liquid, you might be interested in CE4 or EVOD. Both are links to our reviews.
To find out about even smaller clearomizers the size of which matches that of CC510 but are better, you might want to read Panda Evolve Kit where we talk about Kanger t4s and Vision Nano.
basically it's 10 000 draws or at least so they say. I used to replace it every 10 days, but according to this it could last up to 20 days, depending on how much you actually vape. Fact is I can obtain 5pack for the equivalent of 10$ from my local supplier, so I didn't really care.
While refilling, make sure you tilt this clearomizer to one side a little. Otherwise it might leak (not every time, but often enough).
Refilling is simple. Like with any clearomizer of this size, use a needle bottle cap you can buy with your supplies, or a syringe, screw it onto the bottle and remove the top of your clearomizer. Now use the needle to insert e-liquid into the clearomizer through one of the side holes and fill it up but not all the way to the top.