The experience in UI is not only about fonts but also about the encapsulated information . UI should always be looked at as a tool to deliver information.Whenever a user interacts with UI there is a delivery experience  from the visual angle and there is a consumption experience from the information angle.

Delivery experience includes fonts , typography , look and feel as well as colors and this  is a feel good  factor that motivates users to engage more. When we deliver the information in an appealing way that subconsciously attracts users just like the way you get attracted to that beautiful girl in college without speaking to her :).

Consumption experience includes the information that is being delivered through the UI which should satisfy the need of the users. Someone mentioned in an earlier comment about Naukri.com UI being sick but still people came back to them. That is because the consumption experience there is really strong even though delivery experience was very weak. The information that is being presented to the user has to be accurate , relevant and should useful , if not complete.. We have done lot of experiments around this in Homeshop18 and you would be surprised to know that there are users who mail us with some finer details of products that we omit in our general description template.Pluggd.in is an example of a site with very good consumption experience

Both has to work well for a user to be satisfied. If you are weak on either delivery experience or consumption experience users may not get that WOW factor. People can match you in the delivery experience but if you have good consumption experience people will keep coming back. For Eg: Amazon gets 5+ million monthly uniques from India while leading sites in India has only 2+ million uniques. Simply because the value of recommendations and reviews in amazon is priceless (read consumption experience).

So all designers who are reading this - please ensure you capture both in your design. Most designers just focus on the delivery experience and forget about the consumption experience  :). After all sometime you run away from beautiful girls in the college when they open their mouths :D

@ashish few years back Orkut was best fit in that category. I didn't liked the UI (my personal opinion), though I used it becoz it was the only good option available for social networking. I tried facebook in early stage also (when It was not so popular in India) and didn't used it till next 8/9 months until they came up with better UI and functionality.

another example is group dealing sites, so many sites now a days and whenever every new site available, I just see how different it looks or if it is a clear differentiator than other dealing site at time of my first interaction and first interaction means usability, UI. if I don't see UI looks good than trust level automatically decreased.

another example is our IRCTC site (UI was bad for long time but we don't have other option so we still use it)

on another note, I read game2win 's founder's article on bad UI of their site someday back, where he said, people love it though UI is bad so they are not changing anything in existing UI, bcoz it works for them and it works for site visitor, who already know why they are visiting the site

however my comment is mainly for "new product", where first point is to
connect a user as fast as possible to what site claims. a non-techie
user will see mainly for UI first (again my personal opinion), if UI is
good, clean.. site is good usability wise then it helps to establish a trust level at first point it
self.

I may be wrong at some point but bottom line - UI, usability play a major role.

Good UI is must. It's like first impression is last impression. If site looks good, visitors will look for what it does, once they clear on what it does, they try using it to see.. if it does well what it claims. If a product is new (specially web based) then UI is first point of interaction. For me, if I don't see UI appealing, I don't use site again unless it's providing extra ordinary functionality which no body else does.

Typography is a important part of any UI design, it's not only about choosing the right fontface there's much more to it. Sometimes they say 95% of webdesign is Typography. Since, we communicate with our customers through information or content on our website a good typography give them nice feel and they spend more time on our site. But typography takes a lot of time and we need to experiment with different fonts & arrangements.

I used to take inspiration from sites like ilovetypography.com and alistapart.com. But still a lot to learn. It will be interesting to know from UI/UX designers about their take on opensource fonts like Google web fonts, @font-face etc.,