I grabbed a set of the psychic power cards last week and had the chance to use them in a game this weekend.
Until I completed my first psyker, I had only ever needed to use the Tyranid psychic powers, and I was familiar enough with them to simply write down the powers I had rolled, and then reference my rule book. I don't normally buy into the extensive and over-priced ancillary items GW provides, but, with my new librarian waiting to be fielded, I needed to familiarize myself with the other focuses. I quickly realized that there were nearly 50 powers, each with unique rules and restrictions.
I'm happy to report that, while as over-priced as all GW product, the psychic power cards are actually very helpful. Having chosen telepathy, I rolled and selected my cards. Seeing the cards lined against the table edge reminded me not to forget the new psychic phase, and allowed me to see all the rules and requirements of each of the three powers I had at a glance - aiding in the pace of the game.
I also found it helpful to place the active powers next to the units that were affected. Often, when a unit is buffed, I forget to take advantage of the bonuses on them. The cards make for an unmistakable reminder. This relates to the single downside of the cards - they're too large. Frankly, I'm not sure why GW made them the scale that they did, as the information included on each would have easily fit on a traditional sized card, and would have made a smaller, more attractive marker next to affected units.
If you run a psyker in your army, I suggest picking up a set of the cards, especially if you can find them after market, at a more reasonable price.