The good: The extension is updated continuously and very well-maintained by the developer. I love the fact that SEF URLs are generated automatically and efficiently. In essence, you can activate this extension with little to no configuration and let it take care of the rest. It is great at finding URLs you didn't even know existed, and most of the time generates truly SEF URLs. I also love how easy it is to manage META tags.
The bad: The interface is not the most intuitive and has a bit of a learning curve to it, but overall is not too difficult to get used to. It desperately needs a 'Quick Edit' URLs feature as well, similar to their META tag management feature.
I think the worst part of this extension is the customer support. I lurked a lot more on the forums than asked questions myself, mainly because most of my questions had already been asked. What I saw was a lot of pointing the finger at other extensions for issues that sh404SEF should be responsible for. Sure, you could contact the other developers and ask them to help you change how URLs are generated for that extension, but isn't URL management the entire purpose of sh404SEF? It often seemed like the developer just wanted to wipe their hands clean with multiple issues and mark each of their threads as 'Solved' without really caring whether or not the issue was really solved -- sometimes with issues that were 100% a result of sh404SEF. To me, that's totally unacceptable when you are technically paying for the support.
The extension is a little primitive in the sense that there is no way to control how individual components generate URLs or META tags, other than the standard extensions it comes integrated with. Your only option is to use the extension's own router, which most of the time has limited versatility. You can also manually edit each SEF URL created by that component, which is way too much work.
Conclusively, I do wish sh404SEF were more versatile and straightforward, but if you do not need completely custom SEF URLs and META tags that vary according to extension, sh404SEF is a great value and should more than satisfy your SEF needs.
Thanks for your taking the time to put an extensive review of sh404SEF. It's an interesting one, because you seem to be looking at sh404SEF from a specific angle, as a "SEF url manager".
sh404SEF is not anymore only focused on creating nice urls. You can and you will always manually customize any url on your site, and for the most common extensions, there's strong capability to tailor the SEF url build up. But we've been focusing much more on SEO. For instance, automatic management of duplicate urls created by Joomla is in my mind far more important than anything else, in these times of Panda and Penguin Google updates. That focus on SEO directed the introduction of features such as Analytics, "social SEO" and so much more.
As for interface, it's always a matter of workflow and probably a bit of taste, but as the first sh404SEF for Joomla! 3 version is nearing release, now is probably a good time to review. I'd be happy to hear what you found less intuitive and requiring a learning curve, please feel free to contact me.
Please note that many users performing customization on many urls usually prefer to export urls into a spreadsheet, make changes and import them back.
Now again, the introduction of Bootstrap in Joomla 3 will facilitate the introduction of nice UI features.
As you have seen in the reviews here on JED, support is usually a rather positive side for sh404SEF. It struck me in your comment that you didn't had any bad experience, but only commented on a "general feeling", looking up the forum.
sh404SEF is interacting with a lot of extensions, and it is indeed very often happens that source and solution of issues are in other extensions. I can't recall how many relative urls I've seen used for instance, making images or styles disappear when sh404sef is activated, thus making it look as if sh404SEF is causing the issue when it's not.
Another factor to keep in mind while looking through the support forum is that several major extensions have built in support for sh404SEF. In such cases, URL settings and even suport is to be found directly inside those extension themselves. The first that come to my mind are K2, Flexicontent or redShop for instance.