Maybe it's our own experience, but the "nifty stuff" is quite limiting. We've been trying to get combo pack support (ala Amazon's buy this + that together and get $5 off), and it's been a total NIGHTMARE.
For a basic general cart, it's great. But especially for an e-commerce system that you rely on your store for 100% of your income, the second you want to try anything nifty, such as combo packs, flat rate box shipping (mostly calculations involving if a cart can fit in the flat rate box), coupon codes for only specific products, and random general issues (such as 100 variants per product max, which is annoying when trying to hand-create combo packs with 200-300 variants), I'd suggest something else.
For a basic general cart, it's great. But especially for an e-commerce system that you rely on your store for 100% of your income, the second you want to try anything nifty, such as combo packs, flat rate box shipping (mostly calculations involving if a cart can fit in the flat rate box), coupon codes for only specific products, and random general issues (such as 100 variants per product max, which is annoying when trying to hand-create combo packs with 200-300 variants), I'd suggest something else.