.

e.g. This prototype will let you test the user's response to the design and usability of your web site...

или testing of users's response

Фраза "users's response" встречается в смысле "что думают пользователи, их мнение"
См. Google

Those going to websites on the Net are usually described as "visitors" and the number of "visits" (or sometimes
"views") is considered an important factor. which is why I think "visitor" is more appropriate than "user" here.

First reference:
Visitor Reaction To "Chicken Soup" Forum

On The Edge

I like your creative expression and thought processing. Going through some personal stuff right now and I was glad to get the email about the Chicken Soup project and discussion list. I've started putting some of it in writing. Thanks for being out there on the edge.

Celia :)

I love the information on your web site I just visited. I have a wonderful daughter, Stefanie, 15, filled with spirit, joy, love, spunk, and hormones! :-). Down syndrome is her barrier of acceptance into our society. She has been fully included for her entire school career and every year has been a battle for this basic right. I have never told her of the battle I wage on her behalf but she is old enough now to start advocating for her own rights.

2nd reference:
Canadian Success Stories
... Our 100+ page final report provided Panasonic with diagnostics on visitor reaction to its Web site including ratings of the usefulness of various sections of ...

В среде профессинальных веб-дизайнеров пользуются именно этим термином, user response, user friendliness (а не users'). Кроме того, для определения, как Вы выразились, "удобоваримости" сайта применяется слегка жаргонный термин stickiness.

Examples:

... Optavia conducted the usability testing to determine how well the new site served visitors' needs...
www.optavia.com/cs_wi_tourism.htm

Usability Testing for Web Sites: Hypermedia '95
... not representative of the users of their systems; usability testing ... during the development of a Web site ...
www.indiana.edu/~iirg/ARTICLES/ usability/usability.main.html

...usability testing of new site (during March and April) - test ...
macfadden.mit.edu:9500/webgroup/project.html

and lots of other examples

Maybe it's a bit too colloquial. It's hard to know how to phrase it when I haven't seen the whole sentence, but I thought I'd have a go!