XCode 4 has a lot of awesome new features. I'd recommend starting to adjust to it by watching last year's WWDC2010 videos - they dedicated a lot of time to going through some of the new stuff.
I love the new assistant mode - it makes it really easy to navigate through your interface/implementation files. This mode also allows you to create your user interfaces alongside your controller classes, which makes development both easier and more effective than doing it in two completely separate programs!
The jump bar (found along the top of your source code) is a really fast way to leap about your project, making it really easy to get exactly to the place in your code that you're interested in.
The version control viewer makes it really easy to track changes. I now use SVN for all my projects, even the ones I work on alone because it really makes it clear to see how the project has evolved over time.
There are settings to set how your layout changes and what's displayed when you build, when you run, when you end a run etc. etc. which I'm also finding really useful because you want to see different things when you'e debugging to when you're back in your code. It's really nice that Xcode helps you easily arrange your layout to exactly how you want for different scenarios.
The code templates (which can be found in the toggleable right-hand-side toolbar) are also really useful, and you can create your own in order to quickly get code you often write down.
Some more subtle things that I've grown to like:
The 'Fix It' feature works really well for quickly replacing typos. I find I don't even have to read the correction Fix It will make because the fix is so natural.
Control-I short cut for re-indenting code. This is really useful if you add a new pair of braces round your code and you want to quickly reformat. I don't think there was a nice shortcut for this in Xcode 3.
Hitting the escape key to bring up autocomplete suggestions seems to bring up much more helpful suggestions in C++, and looks a lot more beautiful all round!
On reliability, I've been using a combination of Xcode 4 and Xcode 3 since Xcode 4 first entered beta. Whenever Xcode 4 packed in for me or wouldn't let me continue with what I wanted to do then I'd just switch over to Xcode 3 as the two are completely compatible. What I got was a really nice transition period and now I'm completly using the latest build (first week of March) so I think it's very stable.
For people that don't like change, this update is going to be hard. It'll be particularly challenging as a lot of the keyboard shortcuts have changed to make new for a lot of new features. The most obvious difference that people are going to oppose is the new all-in-one window layout. I'm sure people will get used to it over time and realize that this way of viewing everything to do with your project is much more effective both in real estate and in navigation terms.
Enjoy using Xcode 4, it really is an amazing improvement over the last version, and it continues to add to the awesome time I'm having writing Mac and iOS applications. :D
