Day job wise, it's utterly useless. The huge feature omissions, such as lack of legacy project support, and little feature omissions, such as the inability to tick a checkbox to mark a clip as anamorphic, make this unusable. There's a whole list, and it's been discussed ad nauseum.
Having said that, I imagine that it could possibly be useful as a home video tool - but again, I've taken many of the features/workflows I use in FCP7, and use them in home editing. The difference is that at home, I use Avid Media Composer 5. I wish we had it at work, honestly, but that's beside the point. The point is, when you get used to working a certain way, and you take advantage of features and workflows that are available to you in your professional tools, which speed things up even for home video use, you begin to use those features and workflows everywhere. FCP7 and Avid MC5 both make it easy for me to sort media, to manipulate media, and to efficiently do the things I need to do, the way I want to do them.
Now, again, I imagine that FCPX could be useful for many home users. Users of those new tapeless consumer cameras will benefit greatly from 64-bit processing to slice through those super-compressed formats. The simplified output options and many of the features are things that people will find useful for one-off home projects. It will likely be a pleasant experience for them. For DV/HDV tape archival, or people who are converting older Hi8/8mm/VHSC tapes to DVD, the ability to start editing as the video's coming in will be helpful.
Overall, this program will be good for many people, I'm not denying that. It probably will be used in many offices for companies that put out videos on social media platforms (not necessarily in the video/media industry). It will be used and liked by the iMovie crowd. And yes, it might be used by pro editors working on home projects; but I do feel that once you start using professional tools, and you have your eyes opened to just how powerful these tools are and how easy they make things, you really don't want to go back. The exception is that if the price is right, and FCPX's price is arguably right.
FCPX will not replace Avid MC5 for my personal use at home.