POSIT/Other ITG products

Im new to this forum, and you guys really have some great info. I'm doing a report for my class about Investment Technology Group (www.itg.com) and was wondering if you guys had any experience using their products, such as posit. I understand the basic point of POSIT, to get access to liquidity anonymously, etc. but I have a few questions a) do you subscribe to ITG's products? if so, how much does it cost? b) do you just say like "oh I'm gonna use POSIT for this trade" or do you use it for every one of your trades? c) I know its only used for 4% of the NYSE volume, but are their products (BLockALert, POSIT, Triton) popular and well liked?

Thanks guys, I really appreciate it..