The Sofitel has the best location for sightseeing by far. Otherwise just use taxis as moondog mentioned. Suzhou cabbies are a whole lot friendlier and more trustworthy than the Shanghai bunch.

I would not recommend to use public buses in Suzhou. The migrant workers riding them can be pretty rough in behaviour and they burst at the seams during rush hour. I still do on occasion but only certain lines where I know the clientele is more civil and there is room to sit.