Yes I have done several.  I have an older 18" WSM (about 5 years now).  So one of the first things I did was put a thermometer in the lid.  But since then I used an ET-73 and now I have upgraded to the Stoker since I found myself not smoking as much as I liked since the long overnight smokes were so taxing tweaking the temp since I could not get the WSM to stay stable for long than 3-4 hrs.  The stoker has been great and wish I would have done it sooner.
I have put eyelets in for the temp probe, had the 2nd charcoal grate for several years now, but just put the handles on and tied the grates together with wire.  I recently also started using the clay saucer in the water pan and have been very happy with it.  I have added handles to the cooking section and added the Weber side table.  Finally, not that it is really a mod, but I have my access door installed upside down.
I have been looking at the C&C steel access doors, but have not done it yet.  If I do get the C&C door then I will upgrade the door knob to the new lever style.
I have learned a lot on this site and it has been a great help.  Good luck,
Kg