I'd like to add my comments to this thread, having used eCC/Webgility a couple of times with Ubercart.

1. Their support is absolutely atrocious. Their support techs are very low-skill, and they make very little attempt to understand your issue before responding. Every time I've asked for help it takes several exchanges to get something that's even remotely close to a real answer. And sometimes I've just given up. They frequently misunderstand your question, or give you a knowledgebase answer that's not even closely related to what your issue is.
2. Their Drupal module integration is scary bad. I found a major security hole in it, which I told them about (and even included a patch!) and got no thanks in return, though they did update the module with the patch. In addition, they've got issues in their PHP module that have required me to make code changes to fix, including some broken SQL statements, or uninitialized arrays that were generating errors in watchdog. Also, their Drupal module is advertised as being compatible with D6 and D7, but the actual module they provide is D7 only. You don't technically have to enable the module as eCC boots Drupal on it's own in the connector code, but it still doesn't look right when you get started. I created a ticket over this issue and never got a response.

The idea is a great one. It greatly streamlines shipping for my customers (shipping integration, etc), and it's really helped to revamp their order processes. But the excruciatingly bad support experience makes me shudder when I recommend it, because I feel like there's a great chance we're going to be left out in the cold if something isn't working right. And my customers are doing way too much volume to wait a couple of days to beat the issue into the heads of their support techs enough to get a clear answer.