If you're just purely drop-shipping items, and need to keep your inventory synced with your vendor, this is a no brainer. If you've got to keep up with inventory at both your warehouse (2 of our own in our case) as well as the inventory from your vendor (possibly more than 1), eCommhub makes even more sense.
We had a few issues at first b/c the feed one vendor provided did not exactly fall into one of eCommhubs preferred formats. I was able to cut and paste their feed into a Google doc, or excel sheet, and update it that way, but it just wasn't as seamless as I wanted. I contacted the guys at eCommhub, and they confirmed it wasn't a compatible feed, but that they'd take on the task of automating the task for a small fee. It did turn out to be quite a small fee, and the results were quick and amazing! I had little problem with the fee, but if it was offered for free to paying customers, I'd argue many customers would hang around much longer. Hourly updates of inventory from both our warehouses as well as from my vendor keep us from having back-orders. If you're selling online, you know back-orders are not a fun thing in the world of retail e-commerce, especially if you have not provided previous notice to the customer.
As far as support goes, I've had a much different experience than the what Chad has had below. I've been a customer since June 2013, and have had nothing short of a positive experience. After I've had an issue (ie: vendor changing feed w/out notice, etc.), or question about a feature, they've done follow ups to insure we got our issues solved.
One of my favorite features is that fact that it can handle multiple sites. We have several sites, all with many of the same products, and it keeps them all in sync. Beautiful!