I have been daily driving a Porsche for the last 10 years (993,then a 996 and then a Cayman S). I just sold my 07" CS and got the 135i. Apples to oranges. The 135i from BMW (no suspension modifications) is nothing like my CS. The Caymans S was just the best handling car I have ever driven with a completed sorted out suspension that was a street legal go cart with unbelievable handling, steering, braking and interesting enough great ride (disclaimer is I did have PASM with 19" wheels). In contrast the 135i feels like the suspension set up was an aftermarket after thought (most disappointing part of the 135i IMO). It is both harsh and soft with lots of body role. There is also an unplanted/squirmy feeling that is not confidence inspiring under hard acceleration. The weight different and balance are also really noticeable when compared to the Cayman. The Porsche is all sports car.
That said the the engine is great with lots of torque down low. The 135i engine runs out of breath around 6k though unlike the Porsche flat 6 which just rev and pull all the way to the redline. This isn't necessarily bad, it just takes some adjustments to how you have been driving.
In isolation the 135i is better riding and driving car when compared to 90% of the cars on the road. It is a sports coupe and not a sports car. It is brutish and you basically muscle it around. Lots of fun drive. The real flaws of the ride and handling just become obvious when you compare to something like a Porsche. That said the cost difference is quiet large. I also think some the luxury items (i.e. bluetooth, ipod use, comfort access) are better in the BMW than the Porsche. The cabin is definitely quieter than the Cayman. Road noise and mechanical noises are much more isolated and dampened. I like the lack of road noise but miss the wonderful exhaust and intake noise of the CS (I am thinking BMW Performance exhaust to help for me
). Overall I think the 135i is a better daily driver than the CS but it lacks the driving dynamics of the CS.
In contrast to your experience...my Porsche's were rather inexpensive to drive with virtually no issues. I put 42k miles on the Cayman S with no cost other than oil changes (which I did myself). Not one issue with the car at all. The 996 had 78k miles on it when I sold it for the CS and I only had 2 issues (ignition switch and coolant overflow) and 1 brake job (did it myself also). My experience with BMW (wife has always drove a BMW) has been a mixed bag with issues (it has depended on the car). Brake jobs on BMW's at the dealership are no cheaper than Porsche dealership brake jobs...in fact all out of cost expenses at the dealership are about the same for BMW and Porsche in my experience.
I think you will enjoy the 135i just don't expect it to drive like the Porsche (both good and bad).