InfoPath does not use Excel formulas. It uses XPath and XSLT functions...
which are different and there are MUCH less of them.

However, using code, you can accomplish any thing Excel can do -- just not
as quickly and easily as you can in Excel.

If you are looking to collect information from others or allow them to
interact with information from other sources then InfoPath is the way to go
and will be a better environment for doing so.  However if you are doing
Excel core competencies, running calculations on large data sets than
InfoPath may not be the best.
Making the jump between the two, to create InfoPath forms, is not that
difficult but most of your Excel skills will not be that relevant.