It is extremely helpful. It allows you to do most of the important things to protect your Joomla site very easily. It is not a substitute for knowing what you are doing, and it is a really good idea to read the documentation - which thankfully is is quite thorough. But if you have a basic understanding of site security it is very useful indeed. I find the log format particularly helpful, you can see exactly attacks are taking place on your site.
I definitely recommend this extension.
It was very easy to set up and configure.
Something that would make it even more helpful would be the option to individually select for each button whether to show it in the article, category and section layouts. But it is still an excellant plugin. Thanks to the developer.
The only slight niggle I found was that the input text when making a post was very small, really only pixies could read it. It was not that easy to track down the bit of css required to change it. The layout of the files and folders in the extension isn't really standard for a Joomla MVC component, so it makes it that bit harder to find the relevant file. What I found in the CSS is that there are a lot of very specific rules for the font sizes (77 in all!). Some of them use relative font sizes (em) which was causing problems on my site because the font size was already small. If it is really necessary to be that precise about fonts it might be better to stick to using absolute font sizes, then at least there should not be any danger of ending up with unreadable text. Possibly it might be argued that 77 rules for the font size is too many.
But this is a minor reservation about what is a very good extension.
They seem to have ironed out a lot of the more annoying bugs, also they have introduced a system which allows you to extend the main classes through the template, which should allow for a lot more flexibility.
As the creators themselves say, Virtuemart is not intended to be a 'straight-out-of-the-box' complete e-commerce site, you do have to put a bit of work in yourself - or find a good template that does this for you. What it is good for is providing an e-commerce framework that you can customize to do most things that you want.
The annoying things about it (and which prevent it getting 5 stars) are that it is not well integrated with the Joomla menu system, which can cause problems. Also the support is truly terrible. I have never had a question answered in the support forum, really there is no point in even posting.
Having said that, this is a great improvement over the standard Joomla editor. It is particularly useful if you want to add Flash movies, but has many other useful features. The great thing about it is that it allows your to customize a lot of the editor behaviour, though as I mentioned above it only seems to work for the default group.
It also includes a code highlighter, which is very useful if you want to edit the html. Also you can turn off the feature which converts absolute into relative URLs on your site - this is very useful if you are trying to send out a newsletter, otherwise any links to your site in the newsletter will not work.
It is a bit of work to install and set up the editor, but well worth the effort.
It is easy to use and includes plenty of the features that you might want from this sort of component, without being over complicated.
The thing that is really missing at the moment though is some sort of framework for extended field type plugins. If there were a framework for 3rd part developers to create new field types this would provide a huge boost to the usefulness of what is already a great extension.