As with any car they have good and bad. I've had my 03 IS300 for 8 months now and I really love it. It has great handling and with the 5-speed it isn't as slow as the automatic.
If you like to drive hard the inline-6 will drink gas like its going out of style. It wouldn't be hard to hit 17MPG if driven hard on a daily basis. I average about 23MPG and on a few occasions reached 26MPG on a tank. The premium fuel only thing begins to add up as well. Luckily prices are beginning to drop some for gas. Performance-wise the car isn't exceptional at any one thing. In a straight line it would be hard pressed to hang with the new Civic SI (Probably lose to it actually) and the RSX-S would eventually eat it. The good news is if you are an adult, this shit doesn't really matter to you nor does it me. Going around corners is fun imo and the handling is superb. Of course there are better handling cars, but who cares? Its a fun sports sedan!
Now, if you care about all of this, as mentioned by mwmorph, there are ways to improve the capabilities of the IS300. A great website for the modding community is
http://my.is. You can change the suspension, rims, turbo charge, super charge, exhaust, intake, whatever you want to improve the performance. That all sounds good and well but it all costs $$. Especially a turbo. Probably $6-10k depending on what you want.
Now to what I consider the bad. I've already mentioned the premium fuel and weak MPG. Now after I purchased my car I realized that the insurance is super high. Come to find out the IS300 is the most expensive car to insure under $50k or something like that. I have a clean record and good credit and my rate for a 26 yr old single male is around $1600/yr. Also, repairs are expensive as it is a Lexus. I have a Certified Pre-Owned vehicle so nothing has come out of my pocket. I am about to get them to replace my clutch though so its about to pay for itself.
If you have any specific questions feel free to ask. I love my car and it would be hard for me to give it up. I keep my car extremely clean and I enjoy taking care of it. That is saying a lot since I've never taken care of any of my previous cars.
