I decided to install the Insider Preview on my laptop, as a means to evaluate Windows 10 before the final version touches my desktop, and so far I'm liking it.
I, on the other hand, did the same and detest it.
The "returned Start Menu" is a joke, and seems more an insult to everyone who wanted a Start Menu than an honest attempt to meet their needs. You can't rearrange the items on the menu; it's all alphabetical. The text in "All Apps" section is huge and absurdly widely-spaced, making even a short list of apps go on forever. Even if you remove all the live tiles, the menu still takes up a huge amount of space. Its absolutely useless as a Start Menu. Sure, there are third-party alternatives like ClassicStart, but most people aren't going to be using those utilities and I'm going to have to support them.
Then there's the fact that you search your files without the query being sent up to the Microsoft mothership. Searching for sensitive material you wisely stored locally? Microsoft is going to know about it. There are settings in the group policy editor to disable this, but - at least in the most recent preview - they don't actually work. Home versions of the OS don't ship with GPEdit either.
Windows10 is still pretty pushy with getting you into its online ecosystem too, although I will admit it is toned down (ever so slightly) from Windows 8.1. It's slightly easier to notice that you can make a local account without using hotmail, for instance. But from its prominent app-store, to its OnDrive cloud storage, to its mail client that doesn't support POP3, Windows10 requires you to use Microsoft online services to make use of any of its newer features.
Metro, of course, continues to be an abomination, made all the worse by the fact it still remains only half-heartedly integrated into the system. Dig one or two menus deep into the control panel (sorry, its called "Settings" now) and you'll be facing an old-style WindowsXP interface. The shift is jarring and likely confusing to many newer users, and more experienced users will dislike how all the superficial settings have been shuffled about and renamed for no apparent reason. Unfortunately, no third-party app can fix this.
Then there are the niggling minor loss of features. I'm not talking the removal of significant apps - like Media Center - but just little features of the OS that used to be available in older versions of Windows that have been inexplicably removed in Windows10. The ability to uninstall most of the default apps (try removing the XBox app or OneNote; you can't) that come with Windows, for instance. Or control over whether or not to install updates, as another. Individually, these are annoyances but combined they are a headache.
The back-end of Windows 10 seems reliable enough; it has the fast pseudo-start (really, just booting from hibernation since Windows hasn't do a clean shutdown since Windows8) that people like, and seems reliable enough (for Windows). But it doesn't bring anything interesting to the table, still has all the stuff I dislike about Windows 8/8.1 and adds a bunch of unwanted restrictions on top of that. I honestly would recommend Windows 8 over Windows 10 at this point (although if you had the option, take 8.1 or - preferably - Windows 7 if you can). At least with those your computer is still yours to do with as you like, and not as Microsoft thinks you might want to use it.