I recently bought the 650mah Firefly 510-T starter kit from Liberty Flights. I'm pretty happy with it, so far. It was only $40 after discount.
I think it's probably a bit longer than a standard 650mah, but I'm not sure. I do rather like the charge level indication provided by the changing-color LED. The pen light function has proven to be handy a few times for me, already. I wouldn't normally carry one of those around...
When you rapidly click the button 4 times, it toggles it between pen light and e-cig function. It seems that there is no true 'off', though, so I wonder if it'll go dead from the pen light being on if it accidentally remains pressed in a pocket, or if it has a time limit. I haven't tested it. Battery life seems to be right on par for a 650mah, for me.
The 510-T part of the kit is my first experience with
tank-style cartridges. The atomizers work nicely for dripping or with the
tanks. The
tanks are different than what I've seen on youtube for 510-T videos. Those had a hard plastic, hinged cap that had to be pierced, while these have a removable d-shaped silicone plug with an easily pierced center. They look like 901 carts, so I'm not sure if they are 901-T, a new revision of 510-T, or some other aftermarket offering.
The carts seem to work well, but you have to be careful not to push the plug up into the cart when installing it onto the atty. I found that twisting it while slowly inserting it worked well. Filling is as easy as using a syringe through the center hole, or popping the plug off and dripping in.
The one thing that I do NOT like about these, is the plastic smell/taste of the carts. While it's not horrible, it is noticeable. I found that washing them with dish soap before use helps with that. I've been using a few clearos for the last couple of months, and like them better, so I'm not too concerned with the 510-T carts/attys. I'll likely keep them as backups.