The short answer is yes, but with a few caveats. There are two big issues you may run into while upgrading, so we thought a blog post about it would be a good way to inform site owners about what they might run into with the new 3.5 update.
PHP 7
The biggest issue that 3.5 brings is a change in some of the core code to make it compatible with PHP 7. In doing so, some old code had to be changed in the core. If your extensions are written well and follow the current practices, they should just work. There's always a chance though that poorly written extensions or those that don't follow best practices for Joomla might break. The best way to deal with this is to back your site up before upgrading and have a plan to restore it if the upgrade doesn't go well. We'd recommend using Akeeba backup. An even better idea is to use a tool like MAMP or WAMP and use Akeeba to install a copy of your site on your local machine (or clone it to another place on your live server). Run the upgrade on the copy of your site and fully test it. If all is well, then install the update on the live site. This is what we did before upgrading this site to 3.5.
Once you've done that, you can talk to your web host about moving to PHP 7. Some hosts may not be willing to do this, and there are issues if you are running any other software on your server that doesn't work with PHP 7. If you're sure everything on your server can handle it, or if you or your host know how to load PHP 7 for just your joomla install, then go for it. You should see a decent speed boost by switching to PHP 7.
Running the Update
The other issue you may run into is running the update itself. Joomla 3.5 can't be run through the standard extension installer built into Joomla. The 1 click update works fine, but you cannot download the update and expect it to work when you upload it through the built-in installer. We've been told it could even break your site (though this might have been fixed so that it just fails gracefully). So the bottom line is, the 1 click update the Joomla admin prompts you with is fine, but the built in installer system is not. If for some reason you can't use the 1-click install, here's an article on other ways you can install the 3.5 update.
So why do I want 3.5?
Besides the obvious point that it's good to always stay up to date with your Joomla install, here are some goodies that are in the new 3.5 update:
PHP 7 Support
We mentioned this one earlier. It's more secure and faster. If you can move to it, we'd suggest you do it.
Download System & Environment Information
You can now download a file that includes all kinds of information about your site. We can see this being used by extension developers asking you to submit this info so they can help fix an issue on your site. Instead of you having to fill in a bunch of fields, you'd simply upload this file and they'd have all the info they need on your environment.
Article Counts
Now in the category manager you can see not only how many articles are in each category, but also how many are published, unpublished or trashed. This is very useful if you're trying to figure out if a category is empty or not.
Random Category Blog or List order
If you've ever wanted your blog page or article lists to be in a random order, now you can do that.
Editor buttons added to the toolbar
If you're using the standard editor (and not something like JCE), the buttons that had been under the main editor text box are now up in the toolbar (you can see this in the header image of this blog post). This makes it easier to find them and makes more sense for new users.
Easily insert modules into an article
While you've always been able to insert a module into an article using {loadposition} you then had to go into the module and type that position name into the position drop down. It wasn't always obvious and in fact we suspect many people had no idea you could even do this. Now you can just click a button in the tool bar (or under the main text edit area if you're using JCE) to insert a module. A new code {loadmodule} is now inserted with the module type and name.
Drag and drop images into the editor
If you're not using JCE as your text editor, you've probably noticed that you can't drag images right into the editor. Well now you can. The image will be uploaded immediately and placed into the editor where you dragged it.
Other new features
There are a few other smaller features, but the ones we listed are what we consider the most interesting. You can view a list here and an even more complicated, geeky github list here.
In Conclusion
So there you have it. There are lots of goodies in Joomla 3.5 and we feel it's worth the effort to upgrade, as long as you educate yourself on the issues that might arise. This post also wouldn't be complete without a link to the page listing all the volunteers who made Joomla 3.5 a reality. Click here to view that page, and if you happen to know any of them (or even if you don't), give a shout out to them on twitter thanking them for their hard work.