Can I put Debian on my Rasberry Pi?

Yes.

Debian armel will work, but will not make best use of the floating point hardware. Debian armhf will not work (see below).

Your best bet is to use Raspbian - which is Debian armhf rebuilt for the RPi's ARMv6+VFP2 ARM variant.

Should I buy a Rasberry Pi?

There are issues with the Raspberry Pi which means Debian does not recommend it (below). Despite the hype it is a more closed platform than many other things you could buy, and the unusual hardware can be problematic too. We suggest considering one of the many devices in the Freedombox list, or some of the other low-level development boards listed if hardware hacking is your focus

If you really need the hardware-hacking aspect but not full-blown linux, and favour ARM, then these cortex M-profile based MCU boards are worth a look

List compiled March 2013, no doubt many more will be along soon. As you can see there are plenty of things more open, cheaper and more standard than Rasberry Pi's, and which don't misrepresent themselves.

Raspberry Pi issues

The Raspberry Pi issues relate to technical problems, software freedom and hype: