I've looked at both the store module for DNN and I use catalook extensively now - in fact doing quite a bit of custom work with site integration. To be fair, the DNN store module is an excellent start, but if you need specific functions, such as shipping, taxing, options for payment gateways, calendar based sales options, ticketing, downloadables, referrals, vendor based, then you may need to invest in a third party product.
I have worked with Catalook since it first came out in around April 2004 I think, I can't be sure of the exact date. The support from Catalook is second to none in my opinion. And that's only my opinion ok, but my clients who I've introduced to this application have been very happy with what it does.
Some of the sites I'm working with, include architectural hardware, computer store, chalkart graphics for sale, adult products, course booking, stationery online, print and toner cartridges online, printed artwork for sale, dogs and dog breeders and doggy things and will be using it to run my own business products online too.
While the application seems to be overwhelming to some, and in particular those who are selling it because there is alot to take in, but overall, it's a great product with ongoing improvements and support.
While not fully optimised (and my server will be replaced in the coming month) you can view a couple of the sites here -
http://www.style.net.au and http://www.chalkartstudio.com They are slightly graphically heavy - but I do have a few sites coming up that will be very light and fast in design and layout.
I hope this answers some of your questions.
Nina Meiers