Any Tamron/Sigma compare to Pentax 55-300?

I'm looking for a decent tele lens for the Kx. I have the Pentax 17-70 which is a great lens for just about everything. Still though I'm looking for a companion lens for occasional telephotos.

I know the Pentax 55-300 is well regarded (unlike the 50-200) but I would like to go with a less expensive lens if anything compares well to the 55-300.

Specifically I'm looking at these.

Tamron 70-300mm f/4-5.6 Di LD Macro
Sigma 70-300mm f/4-5.6 DG Macro
Sigma 70-300mm f/4-5.6 APO DG Macro

Any comments or suggestions would be appreciated.

Thanks.

Scott

--
Happiness is a want... Contentment is a choice.