To be fair its not that bad, but its definitely a beta and the work of developers.
I found a few issues...
It felt like I had to enter my details at least twice during the sign-up process.
Sometimes you'd click next, and find yourself landing back on the same page and having to click the button again - hoping for a different outcome (isn't that one sign of madness? lol

).
I don't like the Reset Password process, it asks you for an email address (aka account) and for a replacement new password straight away. Then when you activate the email link, you land on a page to "activate" the password - entering the same password which you just picked. (So the only thing an attacker seems to need is access to your email) This I believe leaves your account at risk if your email account is compromised as it doesn't check your credentials further with intermediary questions.
When I logged in a second time, it didn't seem to recognise my existing account, asking me again to subscribe, when I click next it says duh you already have an account - lol - yes, please forward me to the app!
So what was I doing wrong? Well, I clicked the "Start Using" links (under the types of account are listed), and I picked Sage Live Billing. This link doesn't really mean "Start Using" but Start a (new) Subscription... You access the app by the one small link at the very top "My Sage". In that respect I didn't find it very intuitive. In fact this whole page is just a waste of space, the notes gadget is poor, and web search? I mean is it really needed?
It wouldn't be fair without commenting on the invoicing process. I like others found myself wondering how do I update an invoice to say its paid, I'm looking at the Sales Invoices window, but can find no way to update the Status field. The answer is in a separate window called "Receipt Invoices". This felt cumbersome and the process could be much better streamlined.
Anything else? lol... Urm yeah, the New Items (Service or Product) was a bit basic I suppose, but so were the Payment Terms options too.
Good things... Well this list is a bit smaller...
I like the integration with Google Docs, and I like the built in payment processing... But likewise you can see developer "check me out I'm great" bloat here too, as they added Google Docs, but I can't find any worthwhile reason to view or need ("Quick") access to your documents list in a separate window - when you're using a web browser (you can open Google Docs in a separate tab anyway), added to this you have no options other than to open the document. If you could pick a contact, and attach a file (Proposal, Contract or something) then it might have a little value... Anyways, the contact import/export is good.