I've never used the Lastolite cards before, they do look interesting.
The only disadvantage I can see it has compared to the WhiBal is that it is a fabric (kind of a blessing and a curse...). What happens if it gets wet? Water would make it darker (I assume...), so you would have to wait for it to dry.
Unlikely - I know. But it could still happen; rain, snow, sweat, splash of water, etc.
I've never handled one in person, so all of that could be wrong.
Gryphonslair, does any of that apply?
They actually make a waterproof version for scuba diving, film crews etc. It is a well thought out product. From their site:
The EzyBalance ‘point and shoot’ allows you to concentrate on the important part of photography, composition and ‘capturing the moment’. This handy double sided ‘pop up’ 18% grey/white card simplifies the complex technical issues surrounding exposure and colour correction when working in different lighting conditions. This provides a major benefit when working under pressure at events such as weddings or fashion shows. A waterproof version is also available for Scuba Divers.
The WhiBal is also an excellent, well thought out product. Because my main focus is sports I choose the Lastolite for convenience. The large target that folds compact is a plus if you are metering an entire basket ball court for the first time or wanting to meter in a tricky area of a field under the lights and shadows.
The WhiBal is probably the tougher of the two. Harder to damage and easier to clean. If it wasn't for the sports preference I would probably use the WhiBal all the time. I have a small one that I use in the studio as the Lastolite is alway packed in my sports gear that is alwats ready to grab on a moments notice.
If it had been available at the time I bought mine I probably would have gone with this model. Newer and even more flexible in terms of use.
http://www.lastolite.com/xpobalance.php