It's beautiful, and the hilltop perch offers stunning views of the coastline both to the north and the south (though better to the south as trees now obscure much of the northward aspect), and inland towards the Burmese border.
The seated Buddha isn't at the summit but still offers great views to the south, west and east. A couple of drink stalls are here, should you need to refuel. From here, pass by some towering guardians and follow the stairs up to the modern temple -- you can see the spires easily from the surrounds. It's from here that you can enjoy views to the north, both from the railing and a grassy area behind the wat.
Wat Thang Sai is easily reached by hired bicycle or motorbike, or you can walk -- it took as about an hour to walk there from Siripong Guesthouse. Note the upper wat doesn't open until 09:00, so you'll have to jump the fence if you want to wander around earlier. The seated Buddha isn't gated so you can visit that at any time.