I have been using SellerCloud for the last 2 years, and honestly have not had any issues with them at all.
Seller Cloud helped me grow my business above and beyond in many ways. to highlight a few;
A) their shipping software is one of a kind, and is the most flexible I have seen, I can ship orders 4 times the speed as any other software I have used, and has many great features, including rate calculator, batch shipping, upc/item check, order lock - to avoid the same order shipping twice, and the list goes on.
B) the most comprehensive inventory system for multi channel e-commerce, ever since we started using sellercloud, we have eliminated all overselling, yes the system needs to be set up properly with safety nets etc, and you need to keep your inventory accurate, however once that is done, this system will maximize your sales and avoid overselling at the same time.
C) the easiest way to load listings for sales across all channels.
D) forecast purchasing in a flash
E) true cost analysis F) their receiving software avoids receiving the wrong item or wrong quantity.
I have used many software including Channel Advisor, and I don't know how i ever managed my business before I had seller cloud.