We evaluate the performance of the Sloan Digital Sky Survey (SDSS) DR8 redMaPPer photometric cluster catalog by comparing it to overlapping X-ray- and Sunyaev-Zeldovich (SZ)-selected catalogs from the literature. We confirm that the redMaPPer photometric redshifts are nearly unbiased (left angle bracket Delta zright angle bracket) < or =, slant 0.005), have low scatter (sigmaz approximately 0.006-0.02, depending on redshift), and have a low catastrophic failure rate ( approximately 1%). Both the T sub(X)-lambda and M sub(gas)-lambda scaling relations are consistent with a mass scatter of sigma sub(ln )M|lambda approximately 25%, albeit with a approximately 1% outlier rate due to projection effects (lambda is the cluster richness estimated employed by redMaPPer). This failure rate is somewhat lower than that expected for the full cluster sample but is consistent with the additional selection effects introduced by our reliance on X-ray and SZ selected reference cluster samples. Where the redMaPPer DR8 catalog is volume-limited (z < or =, slant 0.35), the catalog is 100% complete above T sub(X) gap 3.5 keV, and L sub(X) gap 2 x 10 super(44) erg s super(-1), decreasing to 90% completeness at L sub(X) approximately 10 super(43) erg s super(-1). All rich (lambda gap 100), low-redshift (z lap 0.25) redMaPPer clusters are X-ray-detected in the ROSAT All Sky Survey, and 86% of the clusters are correctly centered. Compared to other SDSS photometric cluster catalogs, redMaPPer has the highest completeness and purity, and the best photometric redshift performance, though some algorithms do achieve comparable performance to redMaPPer in subsets of the above categories and/or in limited redshift ranges. The redMaPPer richness is clearly the one that best correlates with X-ray temperature and gas mass. Most algorithms (including redMaPPer) have very similar centering performance as tested by comparing against X-ray centers, with only one exception which performs worse.