QuickInbox.com appears simple and to the point as is fitting to its purpose and the logo looks great, pleasing and professional. However here are some minor improvement suggestions.
Homepage could be beautified a little further. To start with, the get/set temporary email areas could be put in their own attractive boxes, with the "or" aligned to the center between them making them stand out more. Here is a simple example to start with:
Something similar could be done on the inbox page to the "Your current e-mail address" part if anything then for emphasis and greater differentiation from the introductory text.
Still, these suggestions are mostly about just further beautifying an already fairly good design.
Design Rating: 9/10
It's hard to get the wrong impression when the name of the site is QuickInbox and the slogan "Disposable Temporary E-mail addresses" which, of course, spell out the focus of the site. There is however a tiny bit potential of confusion with the "get temporary email" and "set temporary email" links in that at first it may seem like both are about the same thing, albeit it is stated that the former is a random e-mail address.
In any case and just to be sure, it may be worth considering a change to something like this. The first could say "Get a randomly generated temporary email" and the second could say "Set your own temporary email address". This reduces the potential of ambiguity and makes it much clearer and more specific.
Focus Rating: 9/10
Aside from the introductory text which clearly and succintly describes the service, it is the service primarily which is the "content" of QuickInbox.com. Clicking on the "get temporary address" is probably an example of the most instantaneous gratification one can find on the web, as I got myself an email address to use for half an hour within literally a couple of seconds.
I then tested this by sending myself an email, and while it didn't arrive immediately the first time I tried (which could easily be due to my own mail server) when it did I could easily read it and even reply to it. Quite impressive. Second time it arrived in a second.
In short this is an excellent service and will probably be quite useful to a lot of people.
Content Rating: 10/10
There isn't much more to add. Design is quite good, albeit it could be made even more exciting, but the service is excellent and quite brilliant so all in all it deserves a top rating.
Overall Rating: 10/10