I've used Firefox's tab groups extensively in the past, but less so recently. Overall, Tab Groups are a nice start but half-baked in their current implementation (as of FF 34).

The basic idea that one can have first-class sets of tabs that are related is just great. You can name them, add or dismiss entire groups. Love it. Tabs are automatically persisted in their groups using the existing UI to add or dismiss tabs. IMO, this makes them far superior to manually managed bookmarks. When resuming a project, I just want to get back to my working state without having had to remember to save every important tab to some bookmark folder. In this regard, I look to tab groups as a memory prostheses, helping me to keep track of what I was/will be doing on a given project between times I'm working on it.

That said, I find that the desktop metaphor aka "messy desk full of papers" simply doesn't scale well for document storage and organization. It never has. It's nigh impossible to organize, refactor, etc. For example, moving tabs that have "leaked" from one project context (tab group) into another, or splitting out a group into a sub-project, is a major pain in the ass. At least Windows Explorer and OS X's Finder have some tools to enable easier drag-and-drop management. Tab Groups introduces the metaphorical constraints, but none of the other affordances of the aforementioned file managers towards allowing organization.

What I really want are organizational tools that let me efficiently manage and swap between various project contexts. Tab Groups have a big advantage over bookmarks here, since tab groups transparently maintain state -- you don't need to remember to save every tab you open to some bookmark folder. You just get your working state back later.

Also, Tab Groups made an incredibly unfortunate design decision: the set of groups is bound to a specific window. There's no way to open Window 1 on Tab Group A and Window 2 on Tab Group B. If you have two windows in flight, each with tab groups, you basically have to keep those two windows open forever (or as long as a window has any active groups). Again, the bookmarks model of a window-independent persistent store is far superior.

Last but not least, the current implementation is hampered severely by slow UI. Firefox opening one window with a bunch of tabs is very slow now, even though tab opening is deferred. I feel like this has been regressing, but haven't rigorously measured.

The proper Tab Group UI is also incredibly slow. It's interesting (by which I mean depressing) to compare the speed and fluidity of Tab Groups to Safari 8's iOS-like tab overview UI. Safari's UI is shockingly fast to appear, and it navigates quicker than a dolphin's dream. Tab Groups is just uniformly sluggish, even when it's not difficult to interact with. It's practically a cliché that web response speed matters enormously, but the Tab Groups UI is so slow that if it were a web page I'd often just give up on it and move on. As it is, I keep reducing my use of it because it's mostly too painful. This comes back to a big failure of tab groups as a memory prostheses -- it's got to be fast and fluid enough that it doesn't become an interference with working flow.

That last element also implies larger feature and performance work I'd like to see. For example, git-quick switching between tab groups. I should be able to type a shortcut, a few letters of a search for a tab group (or a tab search, e.g. all tabs tagged with "rust"), hit enter and immediately see those tabs. By "immediately", I mean "so fast that we need to add subtle animation to the UI to prevent change blindness".