Actually RAB, I disagree.
Not having to suck allows you to maintain normal breathing rhythm and I find that when I'm at threshold, that can be the difference between falling off your race pace for a minute or so, or maintaining your performance level.
I've tried both systems and for me (and possibly others too...) a pressurised system is better.
I just insert the bladder into a 2 litre drink bottle and use that as the containment chamber.
My ski (Stellar SES) has a drink bottle holder built into the cockpit, so exploiting this concept is extremely easy.
Without giving too much away, I know that when someone I'm racing (with a conventional "suck it out" bladder) takes a drink, - that's the time to make a move.
Hasn't failed me so far...