Hondaman, I think your best bet is to post the same question in a BMW forum. The folks over there should be able to give you proper advice on what it is like to own and run a 330i. As to how it drives in comparison to the IS250, I guess it's worth taking one for a spin and letting us know what you think. :)
I completely understand your need for a slightly more agile car. I live in Milton Keynes, and having a less wafty car can be a plus with all the roundabouts over here. My job also requires me to travel to sites that are often located in remote and obscure locations - but they are connected by some really nice (and quiet) country roads, which can be rewarding to drive on in a well-sorted car without ever exceeding the speed limit.
To somewhat answer your original question, I own both a BMW and a Lexus. And despite being the barge that it is, my 745i still out-manoeuvres my IS200. But discussing the driving dynamics of a BMW in a Lexus forum is the same as discussing the reliability of a Lexus in a BMW forum - it often leads to "amusing" replies :P
Let us know how you get on. On another note, did you consider the current-shape Honda Accord as a possible replacement?