Hey plantkiller,
I have the OEM strut brace and like it quite a bit. For the improvement in handling that it brings to daily driving, I'd say it's probably not worth the price, but I did it anyway. I did notice the improvements immediately, which were a more solid feel (less cowl shake) over bumpy roads, and slightly sharper turn-in. The OEM brace is also nice because it does not have hinges, which means it is a functional chassis member and not just a show piece.
I have also installed an H&R 21mm rear sway bar, and posted a how-to in that section of this forum. For the Z4, since we sit so close to the rear wheels, the reduced body roll with the heavier rear sway bar augments the cars handling quite nicely, and IMO is a very welcome effect. My goal of the rear sway bar was to induce a bit more oversteer, which it did. The sway bar was significantly cheaper than the strut brace, however it took me several hours to get it installed, vs. a few minutes spent on the strut brace.
I have not done a front sway bar on my car, but my guess would be that the Z's nose would stay flatter through the turns, but perhaps at the expense of an increase in understeer. Personally, I don't plan on changing my front sway bar, since I don't want to counter the improved turn-in of the strut brace or counter the extra oversteer from the new rear sway bar.
