I went from Z3M -> E46 -> 996tt and Z4M at the same time and now 996 GT3.

Do it and don't look back. Many of my friends from the local BMW club were die-hard BMW guys, and rightfully so, I convinced a few to convert to Porsches and most hate to admit they dont want to buy another BMW or drive their other current ones.

Running costs are a bit higher, maintenance is probably 2-3x as much over the long term, but it's well worth it. I drive mine daily but am now starting to want a daily just for those times sitting in traffic. Make the switch and don't look back.