Last week I picked up the PDF version of USE ME Post-Apocalyptic (UM:PA for short), which prints nicely in booklet format. It can be seen as a revised version of USE ME Sci-Fi, as most of the rules are the same. The optional rule for suppression fire has been added to the book. The main changes come in the form of a much larger list of special abilities for units and a set of force rosters that are good inspiration to make your own units (and illustrate the use of the point system.) On the other hand, USE ME Sci-Fi has the Auto-repair special rule and the option of using an "orbital dart" as off-table support, which are missing from UM:PA. For these reasons, I'd advise anyone interested in the USE ME system for near future, sci-fi or post-apocalyptic games to get UM:PA rather than the sci-fi version. Those who already have the sci-fi version might find enough new material to justify the purchase, especially as it is rather cheap.
How does it compare to Mutants and Death Ray Guns?
A possibly more interesting question (at least for people obsessed with collecting and comparing rules sets, like me) is how does this fast-play skirmish post-apocalyptic game compare to Mutants and Death Ray Guns (MDRG), which has a similar proposal?
The most obvious and possibly most important difference is that while MDRG strikes a balance between close and ranged combat, whereas UM:PA is more of a "shooty" game. While close combat is quite lethal in both games, MDRG ranged weapons have short ranges, aren't particularly powerful, are prone to malfunction and in campaigns require maintenance. MDRG units might end with no ranged weapons at all when using random generation, whereas having no ranged attacks is an exceptional case in UM:PA.
Another important distinction is that UM:PA has more rules to deal with squad movement and actions, while in MDRG the standard "Song of Blades" engine rules for group actions are used. So in UM:PA one can more easily model an organized force while in MDRG it's usually a small band of individuals who occasionally listen to their leader. I believe this is in line with the fact that MDRG is geared towards smaller numbers of figures per side, too.
Lastly, MDRG has more options for movement and actions, and a broader (and crazier) list of special abilities. In conclusion, if your post-apocalyptic world is one of random bands of survivors scavenging few technological relics and fighting small skirmishes with a little "RPG-like" feel, MDRG seems like a better fit. UM:PA can do that too, but it won't feel as detailed; I would say that it fits better a post-global war world where the remnants of organized forces fight for the leftovers.