I was a Luxology detractor, I pretty heated modo workflow (select tool, activate tool, apply tool, drop tool…). But I must admit I was wrong about Modo and his capability. Now I like how it work and how modo doing the job. I like the ultra fast preview and I like basic, but useful 3d paint capability.
I become from XSI, and I pretty think XSI is a one of the best all around application out there, but I must admit the modo approach now intrigue me and I don’t fell any necessity of history stack.
Actually, I hope Modo present a more strong viewport and a roubust CA toolset (just have, but need more efficient deformer). Because, like said, now I like modo approach, and can really enjoy its work flow. About viewport performance read this topic:
http://forums.luxology.com/discussion/topic.aspx?id=61837
is a well know issue, so I think they will work on it.
shader tree appear worse, but it is only the first impression, I think XSI Render tree kick ass modo shader tree and the (worse for me) Maya hypershade, but again, after a bit of practise, I can replicate (except for skin shader where I have issues) all the things I did in Render tree, and sometime in a more faster way. Shader tree has a logic, its personal logic, like Maya Hypershade and XSI render tree, need to learn, understand and respect it, only this 
About stability, upgrade to Sp6, now I have not a single crash. In first three SP I have too many crash, nothing so terrible, but XSI and Maya need an SP in every new release before most critical bug eradicate.
for modelling, I don’t like the modo workflow, like I said, but tools have lot of customization and can fit it to my needs. Agree with generalsketch, what modo doing can be doing with Maya, XSI, blender (with ngons) or Silo, and Maya and XSI (this one IMO is the best) has really powerful modelling tools, but not the some modo feeling, with modo I can doing the some Maya modelling workflow, if I want, or try to a total different approach. Luxology give you a toll set, you can combine tools, setting and customize as you prefer, and don’t need to write a single script, need only to know how work Tool Pipe.