If managed properly- and that is saying and expecting quite a lot-- a modular second story could be dried in the same day as the units are set in place. Modulars can have folding roof sections that are erected in just minutes with just a few rows of shingles to be completed on-site.
To understand the concept, contact a modular contractor and be present while a couple of houses are set in place.

I haven't seen a modular addition, but I have seen a two story modular home. I think one thing to be aware of is how the joining wall will look-- the joining wall is double thickness, and with the manufacturers we looked at, doors in the joining wall were recessed on one side or the other.

I've never heard of modular additions being cheaper than stick. The advantage is that they're supposed to be faster, so the homeowner isn't renting somewhere else for months.

I once saw a modular 4,400 sf home go up. And then down. And then up. And repeat as the assemblers struggled to get the parts to fit together. The finished product had all kinds of detailing problems. Still, in a hot market, the tens of thousands of dollars of EIFS mouldings convinced someone it was a "luxury" home.

Caveat emptor.

Thanks for the responses. My first concern is quallity which is why I plan on looking at more than one completed addition. I will pay particular attention to the joining wall. Looks aside, I'd like to get a feel for how much space you lose from it.

I'd also ask about how they cope with your existing house perhaps being less than perfectly square and level.

I like the cut of your jib, worthy! :-)