The important thing about any accessibility plug-in is having a good understanding about what problems are being solved. When we’re talking about font size changes and narration, these are features that already exist in the browser or in assistive technology – adding this to your website does almost nothing. It may help a small number of people in specific situations, but that’s the limit.
This piece by Joe Dolson (@Joedolson on Twitter) is a pretty good run-down of the disadvantages and general lack of helpfulness an accessibility toolbar provides to any website. Don’t Recreate Browser Features by Adrian Roselli is also another good resource on the topic. I have to concur with Joe’s post, specifically the commentary on how absolutely useless these kinds of toolbars are for people who actually need accessibility. I never use them when I encounter them in the wild, and I’ve never met a person with disabilities who clammors for them, either personally or professionally. I do recognize, however, that, most of the time, they’re installed as a result of the intention to make a site accessible, so I try to balance taking into consideration the good intentions of others with the fact that accessibility toolbars aren’t helpful when it comes to educating others.