Well, one of the cons is that the AT model is particularly thirsty, and pretty slow. But, since you're coming from a 1.6 auto the lack of performance may not be a huge issue.
They are very heavy on fuel though. Don't expect much more than 25MPG unless doing long motorway trips.
I owned a manual one for 4 years and averaged about 30MPG. Although not great, plus £300-ish road tax to boot, I was happy to accept this due to the fact it was such a nice car to own & drive. Plus, the lack of any other repair costs & breakdowns that might occur with a lesser brand made the mediocre economy easy to live with.
However, fuel was closer to £1 a litre at the time than the £2 a litre that it currently costs, so the week-to-week running costs are of added importance today.
Body-wise, the main area of concern for corrosion is in the sills, just in front of the rear wheel. It's a known water trap that will eventually rust from the inside out.
It can be repaired, but if there's already visible bubbling on the sill it's probably too late and an MOT failure waiting to happen.
It's so common that whenever I see a 1st-gen IS out and about, my eyes are immediately drawn to the sills to see if any rust is visible.
Other than that, they are a nice car to drive and own and offer the typical Toyota reliability that you've been accustomed to.
Typical wear & tear components like shocks, springs, brake pads, discs etc are widely available and not hideously expensive, so outside of fuel and toad tax costs, the IS can be reasonably cheap to run.