I traded an '04 Z4 2.5 on my SRT6. There are a number of things very nice with the Z4, but you wouldn't want a base model if you had an '04 or limited.

The new 3.0i has decent performance especially with a stick. Get the auto and it is definitely slower (though the auto has a manual mode and paddle shifters). If you get the auto you will want to shift manually and I used to do this all the time. It actually works better than the Xfire auto, but not the SRT6.

You will probably want both the Sport group ($1K) to get better suspension wheels and tires (16" is standard, this gives you 17") and the Premium group ($3.2K). Leatherette is standard (read that as vinyl) and you will want to upgrade to performance leather ($1K) If you are in a cooler climate the heated seat option will run another $500. The base radio w/CD is OK but you will probably want to upgrade that.

As far as maintenance goes, there isn't any for 4 yr/50K miles. This is very good. They will push oil changes out to 15K miles, but they are free as well. Now about the run flats. The idea is nice and if you pick up a nail and they deflate, you can drive them up to 100 mi at 50 mph. BUT, pick up a nail and you will find no one willing to patch the tire. I had it happen at 1,500 miles and the tire never went flat just 5 lbs lower. I had to get it replaced AND THE TIRES ARE NOT COVERED, NOT EVEN ROAD HAZARD. $250 for an ordinary 16".

Overall they are fine cars and I enjoyed mine until I discovered that to add a little horsepower cost as much if not more than the Xfire. The Dinan stage one kit that added 11 HP cost around 3K.

They do hold their resale fairly well and that is the reason for the good lease deals. In '04 they were $399 per month for 36 months till the end of summer then it dropped to $299 per month for 24 months. The '04 model was over produced (sound familiar) but not nearly as bad as Chrysler did with the '05 Xfires. This car sells between 10-15K a year (like the Xfire does).

Get a macho color. The Z4 has been the number one choice of professional women since it was introduced (several magazines have borne this out).

The 3.0Si will get you more grunt and bigger wheels/tires but will also cost you more (it still won't match the SRT6). But of course you can always opt for an M Roadster for another $10K.

Don't ask me if I would get another one, because I am just too happy with my SRT6...but if I would have had the 3.0 instead of the 2.5 from the beginning, I might honestly still be driving the Z4 with another year left on the warranty.

If you are going to lease, you should probably go with the Z4 (just make sure to option it right) as the SRT6 on a lease will cost MUCH more because of depreciation. However, if you are going to buy and keep it a new (or low miles) SRT6 will be more fun for a LOT less money.