I cannot say enough good things about these guys. The service is amazing, I'm currently on the free one, which is great to get you started. My clients find the service incredibly helpful, because life has a habit of happening and sometimes forget their appointments (how very dare they!! :D ).
Once my business is up and running at a level that I can financially justify a subscription 10to8 is at the top of my list. The increased level of control and branding a subscription seems to enable above the free option can only add to your professionalism.
Oh, & the staff are amazing, they go out of their way to help (but remember there is a time delay due to being on different continents - we all have to sleep sometime!).