My KX 121-3 with thumb gets more hours than my 555E or the
L4630, the usual uses are trenches, ditches & holes then;
stumping (up to 12" stump if bigger the 555's weight & power are called in)
Brush removal, particularly nice if poison ivy is present
rocks
grading
building retaining walls with rock and sometimes heavy concrete blocks (2' x 2' x 4' @ 1600 lbs., use caution! Machine has to be level & stable and you don't have full reach at this weight)
logging
loading firewood
small building demolition jobs
loading scrap steel into dumptruck
many recycling projects, once separated metal from plastic on 4,000 old classroom chairs
disassembling nasty items we usually do by hand but sometimes get stuck with filthy, stinky items that you would rather not touch....
SNOW grooming! I pull a homemade drag and groom 10 - 12 miles of spur trails that lead to our state system of trails. It's slow but after a big storm you can pick up drops and brush in the trail without leaving that nice warm cab!
Getting the other machines unstuck from mud
If I were shopping today I'd get the 6-way blade (was not available back when I bought). If I didn't have the 555, I would consider stepping up one model size to the KX 161 or whatever the equivalent current model is.
There is no comparison in speed and maneuverability when comparing an excavator to a TLB, especially a hoe that attaches to a compact tractor. If that is the only option for some then you have my condolences but my life is too short to be attaching and detaching that hoe and spending all day digging a short trench!