I have to agree on the fact that the ending was bad, but I found the story to be pretty good till the end. Also, becoming a normal guy 6 years later is completely reasonable. The only reason he was able to sustain his life in his room was because he was being supported by his father and mother-in-law. After being only given a enough money to live, he has to learn exactly how to not waste money and only buy the necessities, which he didn't do at all in the beginning and thus got a taste of starvation. After that, everything else comes quite naturally, that is: the need for extra money, getting a job, acquiring a social life, creating relationships with other people, and so on and so forth. Of course he can always manage to get stuck on this path, but 6 years is more than enough time to do this. Now as for how he managed to get his own restaurant and a family of his own, that was probably something along the lines of working at a restaurant, becoming liked by either the owner or his daughter, marry the daughter either through her directly or through her father, and finally inheriting the restaurant - not necessarily in this order... I still didn't like the ending though. It was much too unfair for the woman. I remember bawling my eyes out as I ranted about how it should have gone, but that was quite a long time ago...
Even though this is a 6 year old review. You're very right. I was pretty sad at the ending too. The premise of the manga was stupid. It picked up a little towards the middle-end. But then it just came crashing down again. I'm still glad I read it though. Made me feel crappy -- and when a manga can make you feel something, it's always worth it, right? Main character was a douche. Glad he sort of redeemed himself in the end.