I once asked my sensei this very same question (I'm sure most of us have). He reminded me that it's easy to counter the technique when you know it's coming. We talked for a bit about attacking like I didn't know we were in a dojo and I didn't know he was going to do something. It took some time but when you do there is just no time to let go. He also mentioned (as someone else already has) that if uke lets go...hit 'em! He also pointed out that for quite awhile after you start you are not really doing it at a "real" speed. He then did the technique at the class speed...plenty of time for me to let go. Then he did it at "his" speed. No way. No chance. It went soooo fast there was no hope of letting go in time.
Once when I was on a ladder changing a light bulb I started to lose my balance, the only thing there was to grab to stabililze myself was the puny little string hanging from the light. I grabbed that little sucker for all I was worth. I sometimes think that aikido techniques work like that. If uke has your wrist and suddenly finds himself falling he grabs whatever is (excuse the pun) handy to try and hold themselves up. In this case the hand he is already holding.
Be patient, pay attention and have fun. Don't worry about if it works yet or not because quite honestly it probably won't work outside of the controlled environment of the dojo for awhile. But, with patience and time, it will. Learn the basics...they will serve you well.
just my rantings