I was considering purchasing a SMC PENTAX DA 55-300MM F4-5.8 ED zoom lens but then got to thinking that there may be some more cost effective lenses in the same class from other manufacturers. I took a look at some Tamron and Sigma lenses but I'm more than a little confused at all of the technical specifications and abbreviations. So my question is, should I stick with the PENTAX branded lens that I know will work out-of-the-box or should I purchase a ??-300mm zoom lens from another manufacturer? I'm just starting out in photography and have no allegiance to one lens brand over another, I just want a lens that works well and one that should significantly outlast it's warranty period.

If anyone has any lens recommendation consider that the lens(es) should fit directly onto the PENTAX K-x without using any funky adapters and be available on-line for less than my budget of $350.00 US. For reference, I can get the PENTAX DA 55-300mm lens NEW for $339.00 US and with free shipping to boot!

Looking forward to hearing what you more experienced 'togs have to say.