Is it OK to use switching regulators to charge mobile phones (and such)?
Yes, it's okay to use a switcher, that's what the majority (if not almost all) of cell phone chargers use.  For example one of the Apple chargers is a 1" cube:



To get that small, it has to be a switcher.  Of course it goes from 110 VAC to 5v DC.  To that, it first rectifies the AC to DC, chops it to a very high frequency (100's of kHz) using the switching regulator, and then it is rectified again.  It is much easier to rectify a high-frequency than a much lower one.

In your case, you will be doing the same except you don't need the first rectifying step.

The output of a DC-DC converter will be filtered DC, with minimal ripple (about 150 mA or 3% for the TRS1-2450).

You can also make your own regualtor using a "buck" switching regulator IC, like the NR887D, which accepts an input up to 18V and a output up to 14v at 2A (you will set the voltage of 5v using two resistors).  It is available from Digi-Key for $1.58 in a 8-pin DIP package.



Here the 22 µF capacitors C4 and C5 remove the ripple. 

You need the extra margin on the input since 12v car batteries often are as high as 14v. You will also want to add some suppression circuitry on the input since there are often spikes on the 12v line too.  You can use a circuit like this:



This circuit can be used either in front of the DC-DC converter or the discrete switching regulator above.