I know I use them, and my peers use them, but how much does the average user actually use tooltips to figure out functionality?
Yes people do use tool tips.
For many people tool tips are essential. For example say a person with visual impairments person was using your application. To help them they may be using some assistive technology such as a screen reader.
Lets say that some of your buttons only had icons instead of text. How would a screen reader tell the user what the button did? Tool tips.
This is just one example of where tool tips are useful, there are many more.
It is hard to give a reliable answer for the question, but remember that all users will be 'first-time' users at some stage. Chances are there will be a time when they are confused by a particular button or icon and don't want to take the risk of clicking on it to find out what happens. Also, for experienced users there will be times when they have to access a feature that is not normally used very often. One could argue that since there are frequent/plausible cases for both old and new users to recall information where you don't have space or don't want to apply permanent labels, tooltips are the best way to go.
The only definitive evidence I can share on this subject is from user observations on the application I work on. As I inherited it, it had little question marks near features. Hovering these would bring up the popup. They were ignored by virtually every test I observed. Case in point, I have been on UXSE for almost two years and I JUST noticed the yellow questionmark icon above this form.
What I did do was add small descriptions near features and tooltips that would actively appear to guide users to the next step.
Possibly relevant is the "Old school" UX practice of links and buttons having a title attribute that describes the action the element will perform. The title should be different from the button or link text. I think it's a low level of effort benefit to users to provide them.
Tool tips are indeed helpful when you can not write the label for an icon on a UI. The best example is, in this very page there are numbers vertically surrounded by up-arrow and down-arrow just beside to the answers and the question (like and dislike concept). Here you can not place a label for these icons but a tool tip.
I can't find any numbers on this, but I will tell you that any well-designed application shouldn't leave users guessing as to what the functionality of your features are. If there happens to be something you deem questionable, tooltip.
If you plan on using tooltips, make sure that you use them where necessary (e.g. labels for that element are missing). Also make sure that you don't over-tooltip. Those little yellow boxes (or however you style them) can cover up the content above, below or to the sides and can be annoying at times.
There are NO 'average users'. You need to identify the range of actual and target users for your site or application, and make a design decision based on their specific use of the product. This will require a small amount of research and observation.
No amount of ill-informed 'debate' will give you a useful answer!
The range of anecdotal responses above shows that different specific user groups may have different levels of knowledge, expectation and habit. You need to find out what applies in your own case... while being aware that behaviour may change over time, especially as users become familiar with the interface.
If you do find that one group finds value in the tooltips, then you have to decide whether that value is worth whatever cost is involved (presumably development time and maintenance effort). Again, the answer depends on your specific business situation. What works for one organisation may not work for another, even if they're dealing with the same user segments.
The important point is to get away from the idea that there is a single formula that applies in all circumstances. When you deal with solutions, it's what works in the specific situation that matters.
I'm not sure about the number how many of the user uses it, but me being a power user I consider tooltip is the option which helps a lost. And I would say that now a days when every other design eventually is moving to the Metro design principles of Microsoft the design are becoming more icon centric than the only buttons and links.
There is amount of web-sites a average user access every day is gradually increasing and having tooltips would eventually help any user to reduce the burden to remember the icons.
Tooltips are no more the yellow labels just to show the static information, it can be also used to display the dynamic values as well.
Let's say I want to book an a doctors appointment on his web-site,
For that I have a calendar where I can see which dates are available for booking an appointment. In that case I have three different types of dates which will be presented to me as a user,
1. The Dates which are Available for Booking
For these type of dates an information can be shown on the tooltip as a dynamic value and one a user mouse hovers on the date he will be shown "Available for Booking", this justifies why the the particular date is Highlighted/Active.
2. The Dates which have already been Booked
And for these kind of dates there are different ways to tell a user that the dates have been booked, but again the tooltips can help here be telling the user why the icons are Grey/In Active by telling him using tooltips that the "The Dates have been booked"
3. The Dates when the Clinic is Closed
On these type of dates user can be notified by showing a message on tool-tip which states "We are closed on this Day"
4. And ofcourse Weekends
Here the tooltip message can state that "We are closed on Weekends"
In these kinds of situations Tool-tips are better options and having them show dynamic values helps every kind of user and keeps the confusion away.
But if the web site contains the static information and there are no changes in the value then not showing the tool tips makes more sense. I hope this gives a direction to the kind of answer you are seeking.
It's a topic of some debate, but I do believe there are some valid uses for them in a mouse-based interface. My favourite (and actually useful) use of them, though it's actually rarely employed, is to show the keyboard shortcut for something visually represented on screen.
For example, if a window has a Print button, hovering over that button would show the label (if not already shown) and the shortcut key, e.g. "Ctrl-P". Using symbols for the modifier keys is even classier.
Aside from that, the other very valuable use for tooltips is to expand a long truncated string so the full text can be read by hovering over it.
I actually don't particularly favour displaying contextual help in a tooltip. If the interface is complex enough that it's not obvious what a button or setting does, that descriptive text should be worked into the primary interface whenever possible, not hidden behind a secondary action. Very compact interfaces like a toolbar are exceptions to this, and having a short descriptive text in a tooltip for toolbar buttons can be valuable and intuitive.