I just got back from visiting family in Mérida, and although I had Howell & Webb along, I never bothered to take it out of my bag, honestly. I stuck with H&W for pre-trip prep/study, but during the trip I relied exclusively on the Peterson Northern Central America. For some species which were similar to those included, or in at least one case, where a split (S. coerulescens vs. S. grandis) occurred post-publication, I simply wrote in notes by the appropriate entry. I never really felt handicapped by relying on this volume, honestly, and it's plenty portable for field use. I know this would not be useful guidance for the isthmus or West of the isthmus, but in the Yucatan peninsula I think it's well worth having along.