Jack,
BV Commerce Pro is one of the best .NET carts available and keeps getting stronger with every new version. The support is great, the interface is well done, I just can't say enough good things about it. The code is well written, using proper coding practices and OOP design.
There are a few that I have seen that are just terrible. They might look good in writting but when it comes to actual implementation and use they just don't live up to the hype. I won't disclose them on here as I do not wish to bad mouth anyone but feel free to email me if you want more details.
Something to keep in mind is how well a company implements the features they have listed - So what if it does 3000 things if they don't work right or you can't easily use them. How is their support? Do they have good examples of others using their software?
I took a look at the dotnet cart (I'm commenting on it because they posted here) and can't say much for the online demo. Don't know any stores on the web I have ever visted that look like that demo. Doesn't look like they have a source code version so good luck trying to make any true modifications to it. There were no customer examples of using the cart so who knows how well the design can be modified or integrated. The admin interface was no where as nice as BV's and what about Support?
BV has a nice forum where you can post questions, suggest new features, talk to other developers for ideas, help, modifications, etc. Didn't notice anything close to this on the dotnet cart website. Something to consider.
Anyway, good luck on your search and feel free to email me with more questions...and NO, I do not work for BV.
TIM