GoSystem is clearly the standard. I used it at my old firm, and we use the client version--OneSorce--now. It handles basically everything you can throw at it from a federal point of view (except for perhaps a few rarely used forms, but I can't recall any off the top of my head), and, once you get the hang of it, things work about as you expect.
The "about" is a bit of a concern, unfortunately. It's not always clear where things go, and it's not always consistent from one situation to the next. Where you'll find the biggest issues is when you switch back and forth between Corporations and Partnerships. On a corporation, that check box you need may be found on the "General Settings" tab. On the partnership, it may be down in a completely different section. I found myself on the phone with support more often than I'd like just to root out that check box that I knew was there, but just couldn't find.
As for support, they are top notch...during the off season. It becomes painfully obvious when they first start ramping up support staff during busy season, because suddenly the people answering the phone seem like they aren't even sure what product your using. And, unfortunately, it will often take up to an hour (or more) to get through to anyone, even that useless support person, as the deadline nears.
My last big complaint is that the state support can be hit or miss. Sometimes the states work perfectly well, everything flowing with zero overrides (meaning you have to enter in the data manually rather than having it calculate from another location). More often, though, there's at least a handful of fields that refuse to populate. And support, when you get through to them, will say something like "that's a known issue," or "we'll send it to development." Sometimes the issues get resolved, sometimes they don't. It's even worse if you have to prepare state returns early in the season, since clearly the majority of development is focused on the federal return at that point.
I don't want to sound too much like a complainer, but it seems like anyone should know about these issues before they jump in. From my experience and what I've heard about other software, it's not any worse than anything else in the tax world.
There's really a ton of data for them to recompute every year, so I understand the difficulty of getting it all right on day one, especially when the governments haven't released all the necessary forms. Overall, the software does what it's supposed to to, it's fairly robust, and it does most things well, Just don't expect everything to be smooth sailing.