I might be a little biased, but consider looking at the Digitimer DS3 constant current isolator, which is very popular amongst brain slice electrophysiologists. It also has a unique output discharge circuit that prevents capacitance build up during repetitive stimulation.
I vote with Gareth Thompson . I also have a Digitimer. Mine is constant voltage tough (model DS2A). It's simple, small and reliable. I used ISO-Flex before and due to some weird reason, my stimulation artifacts were bigger on that one. Also, Digitimer gives you some more flexibility in terms of how to set up the pulse duration.
We have years of good experience with the stimulus isolators of WPI (A365). They have a wide rage of stimulus strength and a bipolar as well as unipolar mode. Best, Jakob