In addition to the 4, the WRTA runs service to other surrounding suburbs of Worcester. I'm pretty sure I mentioned that in the 4's post, but whatever. The 15 is another one of these routes, but going to Shrewsbury instead.
The 15 is very similar to the 4. The 15 sees hourly service from 6 to 10 am, and then every 70 minutes until 8:30 pm on weekdays. On Saturdays, it gets hourly service from 10 am to 5 pm. There is no Sunday service. This is slightly worse than the 4 to Millbury with its 75 minute headways. Shrewsbury has a population of nearly 39,000, triple the size of Millbury. At least just give it something on Sundays.
However, unlike the 4, the 15 serves a lot of residential areas. The 4 mainly focused on commerical areas, like the Walmart and the shopping mall. Shrewsbury as well as the part of Worcester along the 15 also have shopping areas, but more people live closer to the 15, mainly due to its travel along local main roads, instead of the highway. In addition, Shrewsbury (at least the parts of it along the 15) is very walkable, and sidewalks line major roads through the center of town.
The 15 happens to serve very key areas for travelers. The 15, along with the 16 and 24, serve UMass Medical Center, which in addition to being a hospital, is also Massachusetts' main public medical school. White City Plaza and Lakeway Commons are strip malls (so not nearly as bad as the Shoppes at Blackstone Valley from the 4) with a wide variety of stores. The bus travels along the residential Maple Ave in Shrewsbury to reach Shrewsbury Center. This is one of those quintessential New England downtowns with some local businesses, a library, and a church. The route finishes its trip at a senior living facility east of Shrewsbury Center.
I do have one major issue with the end of the routing. The end of the route at the senior living facility is 4 miles (or a 10 minute drive) from the Grafton train station. Why the hell doesn't this connection exist? For a town of 39,000 people (and it's a solid middle-class town as well), anyone on transit going east of Worcester on the train has to add upwards of 20 minutes to their journeys by going through downtown Worcester.