PHP's filter functions are really, really great. I've started using them almost any time I need to get input from a user and, personally, I don't think you should use the old $GET, $POST unless you know what you are doing and there is some specific thing you're trying to accomplish that you can't with filter. Filter forces you to think carefully about what inputs your script takes and what format it takes them in.
But there are also some behaviors of filter that can bite you in the rear if you aren't really careful. One of these is knowing which flags you need to pass and what the difference between validation and sanitizing, when is the right time to use each, and what flags to use. I ran into a good example of this today where I messed it up.
I had configured filterinputarray to pull in a variable as FILTERVALIDATEFLOAT, probably because I wasn't thinking like a user and instead was thinking like a developer. I'm the type of person that, when a form wants to know my phone number, I only enter 10 digits without parentheses or dashes. But users are different. They like friendly things. In this case, the user was entering "16,473.54" and the like into that box.
Now, I can look at that and say, "yeah, that's a float" (actually, it's currency). It should be considered a valid value. But FILTERVALIDATEFLOAT will throw this out because it has a comma in it, unless you pass FILTERFLAGALLOW_THOUSAND. Then, and only then, does it return the above as a valid value (in this case "16473.54").
But I looked at the code again. In this case, the value doesn't need to be there except in a specific case, which I handled in error checking in the code, so I switched it to a Sanitize value instead. It's probably a good idea to only use FILTERVALIDATE* functions when your user has to give you a valid value and your script would fail if that wasn't the case. If a validation returns false, you should fail the process and return a (nice) error message to the user. Sanitize functions allow you to accept a little wider range of data and still return a valid value from it. The docs have a great example of this involving email addresses.
So if you're writing PHP these days, definitely use filter. Just be careful and mind the flags and the difference between validation and sanitizing.