One O'Clock Jump was a little too vague with some of the characters' backgrounds, and I didn't find the plot all that engaging. The switchblade-carrying heroine certainly has an interesting past, including a stint in juvenile detention. Some of the gaps were filled in in the sequel, though. Sweet and Lowdown was the better of the two - it also had better historical tie-ins (including Wendell Willkie, the Negro Leagues, and Silver Legion).
Though I can't say I loved the books, I didn't entirely dislike them, either. They provided decent enough entertainment that made up for my cancelled-due-to-avalanche-danger snowshoeing plans.