Planar magnetics design for power electronics naturally involves many tradeoffs, especially in the selection of the core size, winding structure and printed circuit board stackup. Magnetics-in-the-circuit SPICE simulations can facilitate quick magnetics design evaluation and iteration. This paper introduces and evaluates a planar-magnetics-in-the-circuit simulation approach with an M2Spice software tool, which has been developed based on an earlier presented Modular Layer Model (MLM) analysis approach . M2Spice converts the magnetics geometry into a SPICE netlist, which can be simulated with other circuit elements in a power converter under a unified setup. This paper presents an analysis of the applicability and limitations of this approach across wide frequency bands, followed by an evaluation of the accuracy of the SPICE simulation results (by comparing the simulation results to finite-element-modeling (FEM) results and experimental measurements). Multiple planar magnetics prototypes are designed, modeled, simulated, built, and measured, with results reported and discussed.