We've been using QB online for a few years now, overall it is more or less the same as QB Desktop, similar look and feel, however there are a few "bugs" that i'm not a fan of, they used to have an app that you could run, so it was similar to using desktop, however it's now discontinued and you need to use a web browser, as a result, the program logs you out after about 30 -60 minutes of inactivity, which I understand is a security issue, but it's very annoying at times.
Also, even though you can assign different users, all emails get sent from the "primary" user account, so if you have a sales person, bookkeeper etc who sends invoices or estimates, if the client replies to the email with questions, approvals etc, they go to the primary user, not the user that sent the document, which leads to a bunch of un needed forwarding and trying to figure out what is going on. the only other option is to sync QBO to a gmail account and you can send emails from that from within QB, but honestly that's not at all professional looking.
The ability to accept credit card payments directly in QB is a nice touch over Desktop, the rates aren't the best, but it's nice that QB will mark the invoice as paid, email the client their copy of the reciept and deposit the money in your bank account automatically.
Overall I view QB Online as QB Lite, it's more geared to very small businesses with no employees, I think they are just trying to compete with FreshBooks and the other low cost cloud based programs and function is taking a back seat to "cool factor"