Summer time is a time for ramen shops to let down their hair and do something different. The limited edition (限定 - gentei) offerings can be anything really. Strange flavors and unusual ingredients are the norm. And if your shop is already successful, it doesn't really matter if your experiment is a hit or miss.
Gamushara has a definite hit with this one. Ume tsukemen. The noodles are tossed with sour plum flavoring, the soup spiked with chopped up umeboshi (pickled plum), and the whole thing is accented with shiso. These are all fairly dominant flavors, so a weaker soup makes the best match. It is hard to say whether or not you should try this. If you like umeboshi and shiso, the answer is yes. If you don't, then it is a solid maybe.
Awesome twist at the end. Your left over soup becomes ochazuke, a rice dish of a hot tea soup poured over rice. I finish a lot of meals in Japan with a small wasabi and umeboshi ochazuke, but this was a first at a ramen shop.
On second thought those maybes should be upgraded to certainly.