I've been using the 3 larger services for the past few months and each one has its quirks, but I'm reasonably happy with how it's worked out (I think it's not a waste of time). The main limitation for all services is that most big-publisher books aren't lending-enabled.
This is the most transparent site. You can see how many users have listed each book, how many lends requested and fulfilled. Even if the book isn't available, you can use a borrow request
to "get in line" for when someone lists it. You can earn borrow requests by adding, rating or tagging books (it's easy to get them even if you aren't lending anything straight away). This site also gives you a small credit when you lend, and you can redeem this for an Amazon GC once you get to $10 (what you get per book varies a lot, so I don't really know much about how this actually works out in practice).
PS. If anyone wants to sign up, my referral code is AIET4SH7.
This website will try to sell you credits (I find it kind of fishy that they'll sell credits, but anyway, you don't have to listen to them, it's not necessary
to buy any). You can get credits by lending books or by "instant flinging" (this is, you lend the book to a provided dummy email address and then they'll list the book as available for "instant download" by giving out the link)
It's easy to get some credits by getting a few books of the kind that are temporarily free for the Kindle and then (if not immediately, after a few days) you'll usually be able to "instant fling" these books. Credits expire 1 year after earning them.
I like their wishlist, as you can keep track of what books are available without actually requesting them. You can't use credits to "get in line" as with Lendle. If you try to borrow a book and the owner doesn't send it to you, the request gets cancelled after a week and you get your credit back.
This site doesn't use a credits system or require you to lend any books (they mention that it's OK if you are an international user and can only request books, not lend them). You can change the settings to borrow a maximum of 0 to 3 books per week, so you can add as many books as you like to your requests list (like it's a Netflix queue) and not get completely overwhelmed. The priority that's related to you getting one of your borrow requests once it becomes available is supposed to be related to how many books you've lent, but it's not very clear how this works exactly.
I don't like that users seem to sometimes list books and then never actually lend them when a request comes in. I notice this more on Lendle (as there you see how many users have the book listed). If a user doesn't lend a book that they've listed (or removed the book from their available list), then their account should be "vacationed" or something so that the book doesn't keep showing as available when it's really not (on Lendle I have a few books where I've been first in line for weeks when there are copies "available").
Anyway, I hope this helped a bit and that more people share their opinions/experiences! I think these services are great because through them I've also been able to try new authors that aren't usually found in libraries.
Just to be thorough, other similar sites are:
/ Books for Nooks
/ Books for my Kindle
/ Lending eBook