Ken's got a lot of good info there. As a beginner Pentax user myself, I can attest to the value of the system. You can do a lot with a limited budget on a Pentax, but that may also mean an extra investment of time shopping for good deals, learning more about the lenses, and shooting with a variety of lenses that don't all operate the same way.
You mentioned an interest in macro, so I'm posting an example of a photo I took today. Including tax and shipping costs, I used a Pentax 50mm f/1.4 screw mount lens I got off Ebay for about $24, hooked up with a Roxsen screw mount to K-mount adapter I got off Ebay for $16 (I recommend this adapter for a good deal that works well), connected to a Vivitar 2x macro teleconverter I found at an antique store for about $22. That ends up becoming a 100mm f/2.8 macro lens. I don't know what that adds up to as far as macro magnification, but here's how close-up I can shoot with itt:
This was shot at ISO 800, either f/8 or f/11, on a K100d. I could have gotten it a bit sharper if I used a tripod.