What disadvantages are there to the <button> tag?
I started using a diagnostic css stylesheet, e.g. http://snipplr.com/view/6770/css-diagnostics--highlight-deprecated-html-with-css--more/
One of the suggested rules highlights input tags with the type submit, with the recommendation to use <button> as a more semantic solution. What are the advantages or disadvantages of <button> with type submit (such as with browser compatibility) that you have run across?
Just to be clear, I understand the spec of <button>, it has a defined start and end, it can contain various elements, whereas input is a singlet and can't contain stuff. What I want to know essentially is whether it's broken or not. I'd like to know how usable button is at the current time. The first answer below does seem to imply that it is broken for uses except outside of forms, unfortunately.
Edit for 2015
The landscape has changed! I have 6 more years experience of dealing with button now, and browsers have somewhat moved on from IE6 and IE7. So I'll add an answer that details what I found out and what I suggest.
Answer
When using <button> always specify the type, since browsers default to different types.
This will work consistently across all browser:
<button type="submit">...</button><button type="button">...</button>
This way you gain all of <button>'s goodness, no downsides.