TubeBaby

If you want to turn a synth sound into a guitar, run it through TubeBaby. Its as simple as that.
Pretty much anything you put through this plugin will sound like a guitar in your mix. In the past I used to stuff around with all kinds of distortion, trying to get a synth lead or pad to sound a little like a guitar but these days I go straight for this effect and get just what I was looking for almost immediately.
It is not bogged down with controls, although some of the most important ones are kind of hidden away [which is why I marked it down slightly on "User Interface"]. If you click on the "TubeBaby" text in the top-left of the UI, it gives you access to a drop-down menu which lets you choose from several amp and cabinet models, as well as three different "Trebliness" settings. Using these with the front-panel knobs gives you a good variety of tones.
It only has a few presets but it is such a simple effect that a full bank would not necessarily give you any more value. I have not encountered any stability issues and the provided manual is easy to read and comprehensive enough to cover all the bases. I really have no idea about support as I've not had any issues but the manual does contain an email address if you have any problems.
Overall I think this is a great effect. it does what you expect with a minimum of fuss and has the almost unique ability to turn anything into a guitar. The fact that it is free means that everyone should have it in their VST folder.