I don't know where this negativity toward Opel began but they have very good engines and they and the engines are still made in Germany (among other places). The Ecotec engine used in 16valve Opel's are used by all General Motors cars that feature I4 engines, the same engine 2L 16V I4 engine block is used for cars that range from the Opel Astra to the Pontiac Solstice that puts out 194kW. The engine blocks aren't bad and they can be made to put out a LOT of power.
Tuning performance on Opel engines aren't matched by many other engines, C20XE being the most popular with only +-R25k that engine can be brought up to 230Bhp (assuming you get one with Cosworth head, else it'll cost more obviously), which isn't bad for NA esp. if you consider the engines can be had for very cheap locally (I've seen a few people get them for around 3-5k).
As far as know the fastest local quarter mile cars are also Opel, quarter mile in 10 seconds.
I'll admit I drive a Opel, and the new Golf engines that feature FSi are great and everything but I don't understand why Opel has such a bad rep locally, what exactly makes them so bad?