Slash Admin

[slash_mail address="yourmail@mail.com"] - disguises the email address to make it unreadable from mail harvesters and displays it as plain text.
[slash_mailto address="yourmail@mail.com"] - does the exact same thing, but it also transforms the text to a "mailto" link as well.
[slash_mail] and [slash_mailto] (same as above but with no parameters given) automatically retrieve the email address of the author of a post/page. All the above shortcodes use the WordPress' antispambot() function.
[slash_phone number="PHONE_NUMBER" prefix="COUNTRY_PREFIX" text="ANCHOR_TEXT"] - Shows a telephone number in a way that it is clickable. When clicked, if you are on a mobile device it opens the phone's dialer and if you are on a desktop computer it prompts to make a call via a related program (e.g. Skype). The "number" parameter is the only one that is absolutelly required. Here are some usage examples: [slash_phone number="999999"] would output a link "999999" which would make a phone call to 999999. [slash_phone number="999999" prefix="+30"] would output a link "999999" which would make a phone call to +30999999. [slash_phone number="999999" prefix="+30" text="Call us"] would output a link "Call us" which would make a phone call to +30999999.
[slash_home] - retrieves the home URI for the current site. It is the equivalent of echoing the home_url().
[slash_theme] - retrieves the template directory URI for the current theme. It is the equivalent of echoing the get_template_directory_uri().
[slash_child] - retrieves the stylesheet directory URI for the current theme/child theme. It is the equivalent of echoing the get_stylesheet_directory_uri().

slash_dump() - You can use it instead of var_dump() to wrap the output in <pre></pre> tags, for better readability.

My theme also supports some of this plugin's features. Which one should I choose?
It is up to you to decide whether you will use your theme's options or those provided by this plugin. It is recommended, though, that you keep those settings separated from your theme and the reason is simple: If at some point you decide to switch themes, those options will be lost and you have to remember to re-enter them. Keeping them in a plugin maintains the options between themes.
Is it wise to hide update notices from my users?
Generally speaking, no. WordPress' default behaviour is probably the best, that's why the specific option is disabled by default. In some cases, though, users might get confused with those notifications or think that something is wrong with the website. In cases like that, you might want to keep the update notifications visible only for those who can apply them - namely the administrators. Keep in mind that, technically, selecting this option won't remove the notifications for the non-admins - it will just hide them with CSS.
What does hiding options for non-admins means?
Sometimes you only use certain features of WordPress. For example, your website might have comments disabled or not using the Links feature. Also, for better usability you might want to show your users only the options that concern them. Hiding those options won't remove them. You, as an administrator, will always see the full list of all the available options. An editor, though, won't see the hidden options, which helps him focus to only those that concern him.
How does allowing access to appearance settings work?
You can allow editors access to one or more of the following sub-sections of the "Appearance" section:

Customize
Widgets
Menus
Background

Technically, by selecting even one of the above options you give editors access to the Appearance section. To prevent them from accessing unwanted subsections (e.g. you want them to see the Menus but not the Widgets) the plugin hides their links via CSS/JavaScript from both the backend and the frontend. If an editor knew the link for the Widgets subsection he/she could access it. By default the plugin respects the WordPress' default behavior, keeping those options disabled (users have no access at all to the Appearance section).
Old browser warning behaves strangely with W3TC plugin
This is a known issue. When Page Caching is activated in the W3 Total Cache plugin, the old browser warning becomes unpredictable and it may appear not only in Internet Explorer 8 but in newer IE versions as well as in Chrome. To deal with the problem you need to disable either the old IE warning or the W3TC Page Cache option.