if you develop web pages, when will you replace IE7 with IE8 on your windows machine as the default IE browser?
ASAP. You need to test on it, and you do at least get IE7 Compatibility Mode to play with in IE8. Of course it's not quite exactly the same, but you already need a virtual machine to run IE6 in, so what's another virtual machine for IE7? If you are managing to get away without making sites work on IE6 then good for you, but that's far from typical in the industry today.
It's my personal feeling that the way the ‘compatibility mode button’ and the website compatibility lists have been handled is an unmitigated disaster for all parties and the extra complexity for the user will result in irritation and slow takeup for IE8. But still, it's IE, and you're going to have to live with it.
FWIW, IE6 doesn't deserve the “worst tech products” listing. IE6 was a big leap forward when it was launched: it succeeded the nasty IE5.5, and had little competition from the alternative browsers. This was back before Firefox, when all we had was incomplete Mozilla suite ‘milestone releases’ and wonky old versions of Opera. The real problem for IE6 was just that there was no update to it for years, as the alternative browsers raced ahead.
Of course this whole laughable situation with having to choose one particular version of the browser to run, and the multiple-IE hacks, and SuperPreview, and multiple backwards compatibility modes bloating up the browser is all down to Microsoft's original lamentable decision that The Browser Is Part Of The OS.
If we still had a simple standalone browser we'd all be fine; web authors wouldn't need the pain of virtual machines or the unreliability of the multiple-IE hacks, and Microsoft could fix whatever bugs they liked, safe in the knowledge that anyone with intranet apps broken by the fix could just carry on using the old version in parallel with the new.
It may have helped them win the browser war, but the extra coupling baggage caused by this decision is an ever-heavier cross for IE's back.