I second freshbooks. Free for upto 3 clients. Not sure why you think the UI is clunky. I like their simple and elegant UI/UX. Been using it for almost 3 years now. Gets the job done without too much baggage.
I was using Curdbee. They have been acquired by another company called Hiveage, and we had to migrate our accounts there. So that's what I am using right now.
I just needed a simple invoicing app which generates and sends clean invoices, so their free plans work perfectly for me.
Definitely not the best in terms of user experience but it has enough features I didn't know I need, that it keeps me happy.
Things like multiple auto-reminders for late payments and reoccurring invoices saved me alot of time. I didn't need them when I was looking around at invoicing software but now I do.
Solar Accounts - UK specific, suitable for both sole traders and Ltds, with or without VAT registration. Does invoicing and accounting in one nice package.
http://invoiceto.me/ is what I use for sending out monthly invoices. The site is simple, and I keep good records. The only thing I need externally is a pretty PDF to mail to clients, and that site gives me that.
I was using invoicemachine.com but their API just wasn't cutting it for me so I switched to FreshBooks. The FreshBooks API is great: in a matter of minutes I was able to pull my hours from Harvest and generate a FreshBooks invoice. That's all I need
I also use them. I set up an Apache reverse proxy to them from my domain so that their invoices appear to be mine completely. I paid for their freemium upgrades (one-time payment of about $150+). For me they hit the right pricepoint where I'd rather pay than invest my own time on a solution. I wish they supported Stripe or another credit card processor though (instead of PayPal only).
http://www.freshbooks.com/