When a site is difficult to use, investing in visual design will not help. If anything, it will actually make the user experience worse. This is the insight of my master thesis research at Leiden University. 
Beautiful does not always mean usable
15 years ago, a groundbreaking study  hit the ground. “What is beautiful is usable”, the title left nothing to the imagination. This study showed a correlation between the visual design of an interface and the perceived usability. That is, the more beautiful the interface was conceived, the more usable it was judged to be. The interface? An ATM. This study was later repeated with websites and discussion ensued in the scientific community. Because these findings couldn’t be replicated. In some studies beautiful was not perceived as usable, and in other studies ugly could be usable as well. 
Now, with the advent of User Experience (UX), visual design and aesthetics become more and more important as characteristics of good UX. But only adding a nice look and feel does not mean the interface is automatically usable. Usability and visual design are intricately woven together, and one of the threads that connects them is mental effort. In other words, how easy it is for the user to navigate the website and find what they want.
The study: easy vs. difficult
In my research I investigated the effect of mental effort on the relation between beauty (aesthetics) and usability. I created two versions of five different homepages. An easy and a difficult version. On the easy homepages the user could easily perform the task they were given to do. The difficult pages were manipulated so that the given task took some thinking to accomplish. For both versions of the page the visual design was kept the same. 100 participants were assigned to either the ‘easy’ or the ‘difficult’ group. They of course didn’t know there were two groups or that they were assigned to a specific group. I measured how they perceived the aesthetics, their mental effort after performing the task, and how usable they found the site.
The results were surprising. It turned out that when a page was judged to be more beautiful, the impact of mental effort was higher. Between the pages that were easy to use, the more beautiful the pages were judged to be, the more usable they were perceived to be. The most aesthetic page was also judged to be the most usable. However, this did not hold for the pages that were hard to use. For the difficult pages, the most beautiful page was not judged to be the most usable. It ranked lower in usability than less beautiful pages. This implies that a site that is difficult to use and doesn’t look good may rank better in usability than a site that is difficult to use and looks fantastic.
Conclusion: make it easy for your customers
Good visual design means a good first impression.  That will keep customers on your site. But to motivate navigation and interaction, you need good usability. Good usability means that your customers can find what they want, do their transactions smoothly and are likely to return.  A well designed website leads to high expectations. If these expectations are not met, disappointment and frustration ensues. When you make it difficult for your customers, it ruins the first good impression of your beautifully crafted site and they drop out.
Thus, a beautiful website does not guarantee high conversion. Invest in ease of use first, and then in visual design. Then you will reap the rewards of higher conversion.
 Ourtane-Krul, P.J.M. (2011). What is Easy and Aesthetic is Usable. Master Thesis – Unpublished manuscript, Leiden University.
 Tractinsky, N., Katz, A.S., & Ikar, D. (2000). What is beautiful is usable. Interacting with Computers, 13, 127–145.
 Lindgaard, G,. & Dudek, C. (2003). What is this evasive beast we call user satisfaction? Interacting with computers, 15, 429–452.
 Hassenzahl, M. (2004). The interplay of beauty, goodness and usability in interactive products. Human-Computer Interaction, 19(4), 319–349.
 Lindgaard, G., Fernandes, G. Dudek, C and Brown, J. (2006). Attention web designers: You have 50 milliseconds to make a good first impression! Behaviour & information Technology, 25:2, 115-126
 Flavián, C., Guinalíu, M., & Gurrea, R. (2006). The role played by perceived usability, satisfaction and consumer trust on website loyalty. Information & Management, 43, 1–14.