Yeah it can go pretty fast. How ever newer Dewars should be good to go, but these things get pretty beat up.
If you do get a big one make sure you get a hose and defuser. No defuser will equal a lot of gas and will waste a ton of liquid.
http://www.youtube.com/user/BuckeyeV.../2/JveF60WmiP0
Here is the 160L which did not have a defuser on it, freaking Airgas here goes... oh we dont have those, you will have to buy one. I said screw you guys I am not getting anymore from you, so the next 230L magicly had one
Cpt. Cool Himself here Guarding the Dewar with his really Mean Looking Face. I actually had to tip him to let me get any LN2 out.