A rule of thumb that my dad gave me was - If someone needs you to "say yes today or this offer will be gone" then the answer is no.
Whenever I've followed this advice, I've come up on the winning side. You know who uses the now or never technique? Time share salespeople, used car salespeople, club salespeople, job recruiters, and the like. Fundamentally, the now or never technique is used because the people offering you something know that if you had the time to figure things out, you would realize nothing is as good as it seems and those "can't miss" offers are in fact - too good to be true.
If it's a job - then they don't really want you.
If it's a membership - then they are trying to sell you something you don't really need.
If it's a car - then they are trying to screw you.
If it's a timeshare - then find any way out of that office... threaten to call the police... RUN FORREST! RUN!
For a limited time only = crap.