Install is incredibly easy. You do need access to under the car. My mechanic did it for me as I did not have access to ramps/jacks. Took less than 20minutes. Note mine was just lever. There is a DIY on this forum for this install.
Quick Review bmw ssk for 325i (i'm pretty sure lever is diff for 335is) - I instantly felt the throws much much shorter and precise. Big plusses. Effort is slightly harder and notchier but i like this feeling of the shifter 'clicking' into gear.
Something thats interesting is that prior to installing the ssk, i had the zhp and the performance shifter. Both much better than the stock knob. I prefered the zhp knob over the performance shifter when paired with the stock lever. With the ssk lever though, i prefer the performance shifter (ie. the whole kit). Main reason here is that the 1-2 shift with performance knob gives me a more comfortable grip for that 1-2 shift motion.
I've done a few of the stick shifts mods now - cdv delete, ssk, clutch stop, shifters. All +1 and should def be considered esp considering the cost to improvement ratio.