I've been using ASC for well over a year on
HipKraft.com and it is my ideal solution. Trust me I did my research first... product-based stores like 1shoppingcart and others just don't have the flexibility to handle my relatively complex product attributes.
I needed a system that fit my business rules,
not the other way around. For any given shirt design the customer can select the style of shirt, the color and the size - what colors are available depend on the style, what sizes are available depend on the color and pricing depends on the total combination. No other system could handle the multiple levels of logic like that. This is done by Javascript on the front-end but ASC is set up to handle apparel, as it has specific fields/attributes for shirts like size, color, etc.!
Also, I don't want a product-based system! (defined as one where you load your SKUs/products into the system). With 50 designs, many with design accent options on a combination of over 80 shirts that is several thousand "unique products." Some systems limit you to 10,000, I'll be there soon!
ASC is ideal because the programming is up to me - I just submit a form to their cart following their requirements/field choices and it takes care of the rest.
The system handles quantity discounts, affiliates, Google tracking, coupons, multiple shipping options, custom UI, etc.
I currently run two ASC carts, one for my retail and one for wholesale, because the customer requirements are a little bit different. Much easier this way to separate them.
Also the developer is highly responsive. I have worked with them on a monthly basis to tweak my cart as needed, get new features in, better understand some key areas of the system and really make it great. The personal attention and prompt response is unlike anything I've experience before.
I use Paypal Website Payments Pro and the customer has no idea Paypal is in use unless they want to use it to pay. Also integrates nicely with Google Checkout.
I can export my orders into my shipping system in about 3 clicks.
All in all, Asecurecart is flexible, easy to use (for someone who knows what they are doing programming wise), inexpensive and most of all it WORKS.