Vivaldi was a pleasant surprise. 4 OP FM synths can sound weak and boring but Vivaldi - in particular because of it's user interface - is easy to program and creates interesting timbres.
It has a fairly warm sound (for a digital synth) once you start working with the various tuning capabilities and can create fairly complex motion type pads along with good FX and simpler FM style sounds.
The down side is CPU useage which is pretty high when using up to 4 operators. It should be noted you can use as little as one although you can't expect much diversity unless using a minimum of 2 operators.
The presets are ok but you will be better suited programming your own.
Don't let the FM tag scare you off. The programming features are fairly narrow but in a good way.
It's free, it has an interesting sound to it.