They are most of the time interchangeable. There are some drip tips like the "big belly" design and a couple others that don't fit some of my 808 cartos nor do they fit some my some of my 510 cartos. I've had to go out and buy blu cartomizers at Walgreen's in emergency situations and it is really hard to fit drip tips into those tiny suckers. Kind of frustrating but most of the time, they'll work out. I have actually switched to just soft drip tips now as I was chewing on them so much I nearly broke my teeth. You can also buy o-rings at your local hardware store to try and adjust any drip tip to fit any cartomizer. Check out this thread if you have any issues:
http://www.e-cigarette-forum.com/forum/new-members-forum/242980-drip-tip-o-rings.html#post4651435
My first kit was an 808, then I bought more 808s from other companies. I became a bit annoyed that none of the equipment was interchangeable with each other (chargers, usbs and so on) even though they were all 808s. I decided to buy a couple of mega 510 Joyes and a mega Joye charger (got a great deal on all of them) so I could start a collection of Joye 510 equipment and didn't have to commit to one vendor! I like them a lot, but am getting ready to give them to my sister to get her started. There are usually plenty of deals out there for Joye batteries, usbs and chargers. If I were looking to buy, I could just go where the Joye sales are that week and not have to worry about wanting to try another vendor's battery, but then having to buy a usb and a charger for it as well, so I wouldn't void my warranties. I really, really wish their were more groups of compatible 808s like Joye has.
I have 5 non-Joye 510s that I bought from different vendors but these batteries were out of the ordinary so I knew I was buying the same exact battery just in different packaging. When I got both sets of the chargers and usbs and compared the numbers, they were exactly the same. A google image search and sometimes a little further investigating will usually tell you where a vendor gets their batteries from.
Sorry this is so lengthy but thought you might be able to use a few tips I've picked up here! Good Luck!
