I have an Impact DL150 from B&H, 1700 mAH. Mixed reviews on B&H. I've not used it long enough to determine use-length or long term robustness. But I've used Impact li-ion batteries in field audio recording gear for years with great success, so thought I'd give it a try. YMMV.
I have the Impact battery and the original Pentax battery. They both charge fine on the Pentax Charger. They both last a long time. I can't say which will give more shots but they're both up there.
Brian
I sent my Impact battery back to B&H for a credit towards the Pentax OEM battery (the Impact stopped holding a charge pretty suddenly--that sucked at Sea World!) B&H were great as usual.
purchased in Dec 2006 with K10D (2 batteries)- outlasted my K10D and still going strong with my K20D- so no problems here. As of capacity no difference for me (at least noticeable)- and here in pricey Europe 2 spares for one genuine price- why not?
Best and happy recharging, JR