Hi Alex,
At first glance I quite liked the website, looks clean and simple and professional so first impressions were good...but...
The homepage doesn't quite explain to me what your service is. The blurb at the top says its a great low cost way of booking appointments - great, but in my mind though outlook calendar is also a great low cost way of booking appointments - so I need to know more, and I need to know it quickly.
At this stage I have no idea whether the website is targeted at companies or customers or both.
Next thing my eyes are drawn to (and I would imagine most peoples eyes) is the 'Search For Providers' box and Google Maps. I don't know what this is and even if I did I don't want to do it because I want to know what your service is, why it benefits me and why I should want to use it over Outlook!!
It's only after this I noticed the Find Out More button which explained to me what your service does. I think this content should be on your homepage and 'Search for Providers' should be a link in the menu instead.
It's only after reading the content that's NOT on your homepage I understand what it is you do, and I'm still not entirely sure I've got it now!
Part of that is my nievity in this market I admit, but I still think it's something that needs to be fixed.
Other than that the technical side of the website is good, doesn't quite validate with W3 yet (link below)
[Invalid] Markup Validation of http://www.openyourdiary.com/ - W3C Markup Validator
But that's nothing that can't be sorted either, it is beta after all!
I like the design of the website, and my eyes are drawn into the page content which is great (apart from the homepage of course

)
The Features & Benefits page lists loads of features but some links/notes would be useful so I can read more about the particular feature I'm interested in and why it would help me.
I think a couple of minor changes would make the world of difference on this site. When you become involved in a project it's easy to forget that your target market aren't necesarilly going to know that you're targeting them!
Hope that helps, sorry if I've come across as over critical, but if I didn't like the idea and the website in general I wouldnt have bothered posting such a long review
Cheers,
Matt.