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There are two kinds of Rootrainers available online. The primary difference is one of features, rather than of the idea itself, and this appears to be a function of where they are made. Both kinds have so-called "books," which are sheets of molded plastic that fold closed (like closing a book, therefore the name) and create four relatively deep pots. (If you visit the website, this description will become immediately clear). I quite like the concept--it allows you to check root development of seedlings and cuttings, without disturbing the roots, and the grooves in the sides of the pots encourage the roots to grow downward. Since I live in the desert, early development of deep roots is key. There is an open space at the bottom of each pot to allow for drainage, and also results in "air pruning" of the roots once they reach the bottom, then forcing them to branch out in the soil mix. At planting time, just pop open the "book," and you have four plants, ready to go into the garden with the minimum of root disturbance.
As with most things, however, the devil is in the details. The superior Rootrainers are apparently made in England, and I've only been able to find them through Thompson and Morgan. What T & M actually calls the "64-cell model" is two 32-cell models--which is fine, that's a nice convenient size, and it's less expensive to buy it as the "64" than to order two "32s." The "books" slip into an open-sided, open-bottomed plastic frame that holds them closed and upright. Over it all goes a clear plastic "propagating lid." It is this lid that makes much of the difference, in my opinion. Pop it on over the top when the seeds have just been planted, and it helps to keep them moist. Once the plants are up a bit, then put the frame in the tray, and it provides a perfect reservoir for bottom-watering. With the lid, the Rootrainers also work really well for starting cuttings, too--it keeps just enough moisture around them to keep them from wilting, but not enough to have them go bad.
The other version of the Rootrainers is made in Canada, and is available from Park Seed. It is nominally cheaper, but a great disappointment. It has the same "books" (although the plastic seems a little flimsier) and a solid-sided frame that snaps together easily (and unsnaps a little too easily, too, usually when you're in the middle of moving a full tray of seedlings.) The major fault with this version, however, is that it lacks the wonderful propagating lid. This is especially important in New Mexico, where even in the greenhouse, seeds or seedlings can dry out far too rapidly, and die. It also renders the Rootrainer much less useful for propagating cuttings.
All in all, I've been pleased with the British version, and highly recommend it. But make sure you get the one with the propagating lid!