Users can enable the preview pane from the Gmail Labs settings page,. It doesn’t force you to adopt the new preview pane all the time. The team has added an option button to the Gmail toolbar that allows users to toggle the preview mode off and on. It also has the option of choosing between a vertical or horizontal preview panel screen.
After checking mail over the last 20 minutes or so, I’ve noticed that the new visual feature is just as quick as desktop e-mail clients. The only pause is with a 3-second delay in marking a conversation as read after viewing it in the preview pane. However, if it annoys you, there is a setting to remove the delay entirely from the general setting page.
In addition to regular Gmail users, the preview pane feature is available for Google Apps account customers. This small visual addition gives Gmail solid footing as a primary email client tool against the likes of Microsoft Outlook and Apple’s .mail app — the two most common email solutions for enterprise customers.