Round soled spokeshaves can be useful for shaping around the dips, handle of a flatbow. Like many other guys have said they aren't too good on other areas unless the grain is straight and free of knots.
There is one good spokeshave on the market, small curved sole, by HNT Gordon of Australia. What makes it good? It has a tight radius sole, the blade is bedded at 60 degrees, rather than 45 degrees or lower so the wood is less likely to tear out. It is also available with a hss blade which outlasts tool steel many times over when working hard, abrasive woods. The blade can be flipped over, which turns it into a 90degree scraper. The only downside is that it is very expensive.
Another good spokeshave like tool, is a gunmakers scraper. Its a scraper blade set in a spokeshave like holder(either rounded or flat sole. Used for smoothing curves in riflestocks. Woodjoy tools used to make a really good one, I don't think he's offering them for sale at the moment, though he might make another run of them if enough people show interest in them.