Having a few spare moments (ha!) this past weekend, I installed the fore-end on the house Mossberg 590. Attachment was very simple - the tool Magpul includes to remove the castle nut works AWESOME. Aside from removing my own WECSOG* additions from the shotgun, it took perhaps five minutes from laying the 590 out on the bench to tightening the magazine cap after installation.
Should accessories be desired, the M-LOK attachment method allows for immediate installation without removing the fore-end. However, it would only take a minute at best to remove if so desired, or to return the Mossberg to original condition.
*Wile E. Coyote School of Gunsmithing, thanks to SayUncle for the term.