I use them all the time.

Several of the features have (kinda) been implemeted in maya 8.5 edge loop and ring select, chamfer verts, etc.

I mostly use the built in maya edge and loop stuff now. Although mjpolytools has more robust functionality.

You can see what all the tools do at this web site:

https://www.maya3d.dk/Scripts/MJPolyTools/

If you want the best maya plugin I have seen consider DRasters Nex for maya. This has the best feature set of any plugin I have seen. Personally I feel all these features should be part of the native maya tool set but I guess hell has not frozen over yet.

Although I take a major hit in efficiency I avoid using plugins. They are not guarenteed to work the next time autodicks decide to change the name of polygons to mesh to numnutgons or whatever and play musical toolbars and then call it a new version of maya. I used to use byron's polytools and was nearly crippled when they were no longer supported, I had become that dependent on them. So my new moto is until they put it in the standard tool kit I avoid it. If I want a good set of modeling tools I use modo.