They sound good, have a very wide range, and are comfortable. How much you like them will come down to what you're trying to accomplish with them, and what you're using to drive them. I also used the SE425s for a while, and they sounded pretty nice too, our other guitarist still uses them.
IEMs are awesome for hearing yourself, great for polishing and tightening your vocals, working through tough harmony parts, etc. They can be difficult if you need to hear the audience (like being at the front of the stage with people walking up to you). You really have to work on imaging and effects to make you feel like you're still in the open air space of a stage, and it can be pretty close, but you never really feel quite as connected. Then again, you also don't feel as deafened either!
Like most things, they have pluses and minuses, and it'll come down to what you want to hear.