I find the trimlines to be nicer to write with than the basic strait bodied slimline. They aren't significantly larger/heavier than the slim, like the Cigar will be, but they "feel" a bit better to me.
The Aero series (Craft Supply, Exotics, and a few others sell them) is also smaller in size than the Sierra series, and I like them a bit nicer than the Sierra. The Aero's also do a better job of making a feminine pen in my opinion than the similar style Sierra Elegant series.
You could also look into the Designer/European pens, which are 7mm cross ballpoint pens but a little bit beefier while not being large. Their profile can be varied a bit to make it lighter or heavier depending on your desire. Another is the Slimline Pro, which is a larger version of the Slimline (different size tubes, 8mm drill bit) and uses the Parker refills (like the Cigar, Aero, Sierra, etc) which, again in my opinion, write better and come in a greater variety of ink types and colors (you know you want purple ink...:biggrin
As to the original question, everyone covered it, but to recap:
Pros: Convienient, precise, quick, no drilling needed
Cons: Expensive, limited wood selection, limited kit selection, difficult to modify, segment, etc.