Can someone tell me a little more about their m series m-lok drop in? what exactly is the m-lok and does it limit any attachments I buy like vertical grip etc.

The "M-Lok" refers to the attachment type for accessories. There are bolt on rails sections so you can use picatinny rail type attachments, but it is designed for "M-Lok" attachments. There are M-Lok light mounts, sling mounts, vfg's, afg's, etc.  These attachments mount directly to the M-Lok slots built into the rail. The M-Lok pattern originates with Mapul and is becoming more common with various manufacturers.

I can't speak for that particular rail, but MI products are usually good to go.

Yes I have one - a 7" M Lok drop in. It is a beauty. Yes you can attach a vert grip (plus much more) with M Lok. It saves weight - as now you are going diirect to the guard - no pic rail needed so it is lighter.