I happen to own the Spyder3 Pro colorimeter, so I tried out the app and can tell you a bit more about it and how it works with the iPad. As I said, the SpyderGallery app is free, but you will need the Spyder3 hardware to make use of it. Any Spyder3 device will work, whether you bought the Express, Pro, or Elite display calibration package. Or you could even borrow the Spyder hardware from a friend.
The Spyder3 normally connects to a computer via USB for color calibrating monitors, but for the iPad, which has no USB port, they came up with a way to read the data from a computer on the same local network. For this you need to download the SpyderGallery Desktop utility for your Mac or Windows computer. This utility provides the WiFi link for your iPad to communicate with the Spyder3 hardware connected to another computer. While you're downloading that, you may wish to view the informative video tutorial from within the app.
SpyderGallery App Home Screen
Once you have the SpyderGallery app for your iPad and the SpyderGallery Desktop utility installed on a computer on the same WiFi network, you are ready to connect the Spyder3 device to the desktop computer and begin the calibration process. The iPad will find the Spyder device attached to your computer and prompt you to connect to it. The iPad will then display an outline where you are to place the Spyder3 device while the iPad is laying flat.
There are no instructions about adjusting the room lighting or the iPad brightness, so I assume this doesn't impact the calibration. In the next step, the iPad will display a series of colors for the Spyder to read and analyze. This takes about 2-3 minutes.
When it's finished, you can use the viewer within SpyderGallery to browse all the pictures on your device. SpyderGallery accesses the default photo library of your iPad so pictures do not have to be imported or duplicated. The viewer allows you to toggle the calibrated view off and on so you can compare the standard iPad photo display with the color managed view of the same photo. The viewer has nice navigation controls, including a filmstrip, and the ability to zoom in to pixel level. On my iPad, the color adjustment was very subtle and I noticed the most difference in blue and red tones when comparing the corrected and uncorrected photos.
The viewer built-in to SpyderGallery allows you to browse all the photos in your device library and see them with a custom color profile applied.
Unfortunately, SpyderGallery is the only app with which you can view the color corrected photos. There's the rub. If you have a portfolio or photo album app which you prefer, you won't be able to see the color managed photos with those other apps, nor will you be able to do seee color corrected images when working with photo editing apps on the iPad. And you're stuck with the pathetic (read: nonexistent) photo sorting and organizing capabilities of the default iPad photos app. However, the User Guide for SpyderGallery implies that developers may be able to utilize the color management profile provided by Datacolor in other apps:
Within the settings of SpyderGallery, you can choose to display a watermark on the photos as you browse them. There are options for a text or image watermark, and settings for adjusting the position, size, font, opacity, and so on. Watermarking is a nice feature for those who will use SpyderGallery as a mobile portfolio.
The full-screen view of SpyderGallery. You can toggle calibration off and on, display a watermark on the photos, and use the filmstrip to view other photos. After a few seconds, the toolbar disappears for a truly full-screen view and a tap brings it back.
The app also has a SpyderProof button, which appears to be just a simulated comparison of an uncalibrated and calibrated example photo. I'm not sure what the purpose of this is, except perhaps to show the potential to users who have not yet acquired the Spyder3 hardware. The "Improve Your Color" button takes you to an advertisement for other Datacolor color calibration products.
Obviously if you own a Spyder3 device, color management is something this is important to you. If you also own an iPad, you will definitely want to get the free app and take advantage of this benefit. If you don't own any color calibration hardware, the free iPad app gives Datacolor a bit of an edge over other products by providing free iPad color calibration.
For me, the difference in calibrated versus uncalibrated photo was not noticeable enough to be concerned by the fact that I could not view color corrected images in my other apps. But I now have the assurance of knowing that my iPad already displays nearly-accurate colors, which makes me happy.