Hi,
I've had some experience simulating planar microwave circuits using Sonnet.
It is fairly easy to learn and use, and is accurate for the most part.
While I have used Sonnet, I'm particularly fond of another software package called IE3D. IE3D's user interface is not as user-friendly as Sonnet's, but gives excellent results.
To perform the electromagnetic simulations, both programs allow you to specify the different layers of your geometry, including the relative permittivity and loss tangent, etc. Then you draw your geometry onto the circuit and assign it metalization parameters such as conductivity.
Once this is done, the program divides up the circuit into smaller "cells" and enters them into a large matrix. Simulating the circuit involves solving this matrix.
Sonnet's downfall is that all of its cells must be rectangular, and equally spaced across your circuit. So, if you're simulating curved microstriplines, some error would be introduced because your circuit would have square edges. To alleviate this problem you would need to increase the number of cells, and thus increase the calculation time.
IE3D on the other hand uses triangles to create the circuit mesh, and allows more accurate simulation of complex circuit geometries. Its intelligent mesh generation will also create more cells on the edge of the microstrip than in the middle to account for the uneven current distribution.
Sonnet has one major advantage over IE3D in one area - dielectric bricks. Sonnet allows you to insert dielectric bricks into a circuit so that different parts of your circuit may have different substrates, wheras IE3D only allows you to vertically stack the different substrate values.
I think that either program would work well for microstrip antennas.
Hope that helps!
Matt