The TWSBI VAC-700 is a pretty cool pen. It has a pretty interesting vacuum-filling mechanism, not all that common in pens today. Doing a 'conventional' fill, you get about 1.5ml of ink in it, which isn't shabby at all (about 3x's the capacity of a standard cartridge/converter). However, if you're like me and you derive a sense of personal satisfaction from maxing out your pen's ink holding capabilities, you can get a full 2.3ml into your VAC-700. (note, the Vac-700 is discontinued, but has been replaced by the Vac-700R)
This technique can be used for any vacuum-filling pen such as the Pilot Custom 823, as well. It does take a bit of practice and some guts to take the risk of shooting ink everywhere, but if you're like me and live life on the edge (not really), you'll enjoy having one of the largest ink capacities around. Let me know what you think!