I just went to Mentatsu yesterday after reading a positive review online, and it turned out to be a really bad experience. I ordered the chashumen, and my husband had the miso ramen. Neither were decent. The soup was very bland. The noodles were too soft. The bean sprouts were definitely not fresh. The so-called chashu seemed like overboiled slices of lean pork. It was so bad that we could not finish our bowls. Moreover, they forgot about our gyoza order, so we didn't get to try that. Having just gotten back from Japan last week, the experience was especially disappointing. If you're craving ramen in the O.C., I highly recommend Kohryu and Shin-sen-gumi--both are 10x better than Mentatsu (in both food and service).
Pros: Prompt seating
Cons: Bland soup base, soft noodles.