Among the Best. Boundary is a sweet canyon tucked between "The Big Ones" up on Kolob Terrace, and is one of the best canyons in Zion. It makes a nice daytrip exiting out the MIA, or a great way to access Kolob Creek for a high-class, technical backpack trip. The technical part of the canyon starts right AT the Park Boundary, thus the name.
In the spring the canyon runs with snowmelt and can be too burly. By the time the West Rim Trailhead opens, it may or may not be down to a reasonable level. In recent years, after big winters, it has run strong all summer, and full wetsuits have definitely been required. In dry years, it may dry out in summer - hard to say when. When wet, Boundary is as cold as Imlay or Kolob - bring plenty of warmth or suffer mightily.
The walk from the intersection with Kolob Canyon to the MIA exit is IN Kolob Creek, so be sure to check the release level of the Kolob Dam by calling the Washington County Water Conservancy District (435-673-3617) AND check the flow where the Kolob Terrace Road crosses Kolob Creek, 1.2 miles north of the Lava Point Road. Hiking in Kolob Canyon with more than 10 cfs, even just to the MIA exit, can be challenging and slow; but there are no technical obstacles so it is certainly possible.
Boundary Canyon is a Class C canyon when wet and Class C techniques should be used. Carry an extra rope in case you get one stuck. Perhaps two.