I'll get right to it: anyone using a mature copy of Windows 7 (as opposed to a new computer or a newly installed copy) should not install Service Pack 1 (SP1).
This is not a knock on Microsoft, Windows or the service pack itself. The Defensive Computing approach is to let the curious, the un-informed and the pioneers suffer the inevitable problems with any major release of software.
I always advise waiting before installing a Windows service pack. Think of it as a cooling off period. Microsoft can't possibly test everything. And the service pack itself is not my big concern. Rather, it's the interplay with non-Microsoft software that accounts for most of my hesitation.
Waiting, allows time for software vendors to discover any incompatibilities with the new service pack. Given sufficient time, affected vendors may include fixes in new releases, make patches available or, at the least, document a work-around.
A service pack contains three things: previously released bug fixes, new bug fixes and new features. Any pessimism needs to be weighed against the improvements offered by the new bug fixes and/or new features. From what I've read, the improvements in Windows 7 SP1 are trivial and/or irrelevant to most users. Thus pessimism wins hands down.
To be clear, my point is to wait, for now, not to avoid SP1 forever.
The decision to wait is, to me, a no-brainer. How long to wait, however, is a matter of opinion. My best guess is to hold off until August 2011, which would give SP1 about five months of exposure to everything the world has to offer. I waited much longer on Windows XP Service Pack 3, but Windows 7 seems more mature.
According to Computerworld, sometime in April the first new Windows 7 machines will ship with SP1 pre-installed.
What about new Windows 7 PCs without SP1?
As with any new computer, you should initially kick the tires as much as possible. Then, when you are reasonably confident the machine is not a lemon, would be a good time to install Service Pack 1. This way, problems surface before you have become dependent on the machine, while tech support is still available and while restoring to a factory fresh state is a viable option.
I have read recommendations that if you are re-installing Windows 7 then the first thing to do on the newly installed copy is to install SP1.
If you are re-installing Windows 7 from scratch, the first thing to do is get up to speed on disk image backups.
OTHER OPINIONS ON WINDOWS 7 SP1
Many others have offered their opinions on installing Windows 7 Service Pack 1. Most suggest waiting.
In the Windows Secrets newsletter, Woody Leonhard recently offered a summary of the new features in the service pack. Summing it up, he says that SP1 "adds surprisingly little to Windows 7" and "there's not a single significant enhancement". He suggests installing it eventually, but not now.
A short version of the Windows Secrets newsletter is free, you need to donate to get a longer edition. Susan Bradley wrote about SP1 in the paid edition, so I won't go into detail on what she said other than to say that she suggested waiting.
Windows 7 Service Pack 1 (SP1) is now available. That doesn't necessarily mean that you should just immediately install it, though ... I'd probably wait a few months, say June, 2011 or so. This is just to let others experience any problems that might be part of the update, and to either allow SP1 to be fixed as needed or, for support sites and the Microsoft Knowledgebase to accumulate any workarounds to problems that people have experienced. I'm not saying that a problem is likely, not at all. But the reality is that a few people will experience issues with the update, and it'd be best to let others blaze the trail, just in case you happen to be one of those few ... For the vast majority of people, the SP1 update should not be an issue. I do recommend that you install it eventually. You just don't need to be in a rush.
In the Win7News newsletter, Deb Shinder said that on whole SP1 was "not terribly exciting" and her summary of the new features was the only one I saw that mentioned support for hard drives with 4K sectors. As for whether to install it now, she said "it depends" but the only argument she offered for not waiting was "if you've had auto updates turned off and haven't been keeping up with the monthly patches, installing SP1 is an easy way to catch up and get your system security up to date."
Brian Krebs said "If you are staying up-to-date in security patches, you are not going to gain much by installing this service pack, which contains a few uber-geeky feature improvements that are mostly a bonus for users of Windows Server 2008 R2 - not Windows 7 ... I would urge Windows 7 users who are up-to-date to ignore this offering, at least for now."
When he wrote about Service Pack 1 being released, Paul Thurrott focused on the process of installing it rather than the timing. When asked, by email, for his opinion he said "I don't see any reason to wait. It's a rollup of previously-released fixes for the most part. There are no reports of widespread issues."
Steve Gibson, on his Security Now podcast, said "there's no hurry about installing it".
Ed Bott has written about Service Pack 1 twice. In the first article he says "Any time Microsoft pushes an update to a few hundred million users, you can expect some hiccups" which lead into a discussion of software to help repair a broken instance of Windows Update.