This subunit has a pretty complex history. In 2008, a one shot unit known as Okashina Sisters was formed to sing the ending theme of the anime Miricle Mimika. It's members included Oota Aika, Watanabe Mayu, and Nakagawa Haruka.
In October of the same year, Akimoto announced he would be reviving Ushirogami Hikaretai, a subunit of his 1980s idol super group Onyanko Club. Ushirogami is mostly remembered for singing theme songs for the anime High School! Kimengumi and only lasted a year before being disbanded. Akimoto revived the group by having Hirajima Natsumi join the Okashina Sisters and renamed them Watarirouka Hashititai.
February of 2010 saw Kikuchi Ayaka increase the membership to five.
Finally, in June of 2010, Iwase Misaki and Komori Mika joined and the group was renamed Watarirouka Hashiritai 7. Most recently, SDN48's Urano Kazumi has been transferred in to replace Natsumi following her scandal and subsequent departure from AKB.
If you're looking for bubbly pop music, Warota 7 is the subunit for you. For what they seem designed to produce, they do an exceptional job. And while it may not be something I'd listen to on a daily basis, there is definitely a place for catchy, upbeat music in any collection. The membership roster is formed from some of the cutest girls in AKB as well. Here's a small sample from their newest single "Shonen yo Uso wo Tsuke!" I think it exemplifies the kind of music one can expect from this group.