I've used their service several times so I can really say that they make an instant payment. Now you have to see if the place where you made the transfer had sufficient reserves. For example, I had to transfer my money from MT.Gox to BTC-e and the last (BTC-e) had no bitisntant-reserve of money, so I had to wait the Support of BTC-e to put more money for everyone made the same transaction(MT.Gox to BTC-e). But this one was my fault for not having seen BTC-e total reserve of bitinstant system. Send an email to them if it's no the case.
https://www.bitinstant.com/contact
For support, please contact
support@bitinstant.com