Is Ultimate Upsell a good app for Shopify stores?

It will really depend on what it is that you are trying to achieve. In my personal experience, Ulitmate Upsell is a great upsell app but is not the best for actual bundles.

I personally prefer the way that the Bold app works and looks on screen. From what I can tell, Ultimate Upsell looks like a good app, but I have never used it.