I give this event a 6/10.
Initially, it is a wonderful event that gives Warframe a new flavor of gameplay. The gameplay is pretty solid with the Soaktron mechanics being realistic to how a real-life squirt gun would work, the artwork of the map is beautiful, the little gimmicks such as Warframes wearing tubes is cute, and Kela De Thaym's drug-like voice acting nails this event into a grand immersion! At a first glance and first few playthrough, this event is spectacular.
I say "at a first glance" Dog Days is great; however, the longer you participate in the event, the worse it gets. Pearls are the new currency types earned in this event, and they are fixed by the mission difficulty, not by the amount of kills gained in each mission. This gives players the incentive to not be engaged in the event and find ways to reduce the amount of work load on their end. This can be done by gaining a single kill, then head to certain spots on the map that would render the player safe from enemy harm such as trees, buoys, and Grineer vaults.
In addition to the reward system, the prizes of the event that can be obtained through Nakak, from Cetus, are quite high. The Captura costs 1,000 pearls, Beach Ball Floofs - though they are a spectacular cosmetic - range from 300 to 500 pearls in terms of pricing, and the Dog Days Sigil costs 600 pearls. These are just a few of the many prizes with inconveniently high prices.
The fixed rewards and the high prices subtract about 4 points from my rating. Should these problems have not existed in the first place, this event would have easily been a 9.5/10 in my books.