I've used a Sony Reader for as long as they have been available (3+ years?). My wife has one, too. We also have iPads.

The Readers have been to many beaches and pool vacations and work great.

The iPad is completely unreadable with polarized sunglasses in portrait mode. Thanks to the touch screen I'd also expect it to get more greasy (sun lotion?) - the Sonys we have and most other dedicated eReaders don't use touch screens.

The iPad has a very good battery life, but an order of magnitude less than an eInk reader. Never any worry I am running out of juice while reading.

I have used my iPad in the sun (backyard) for email and surfing and it is sensitive to overheating. (gives you an on-screen warning).

Sony is coming out with an iPhone (and iPad?) app any day now, so I can access all our books on the iPad (same is already true for Kindle, Nook, ...). That's useful when I go on a trip and dont want to bring along both devices, but for pool/beach vacations I will definitely continue to use the eInk Reader.