Voidwall grenades are great for finishing someone who hid behind cover, and spike grenades can be used pretty effectively for basically everything if you practice their placement. Vortex grenades are rather lack luster... but can help prime a freshly revived enemy if you happen to run out of special. They're only bad when you compare them to other grenades such as fusion or firebolts. Except for vortex. I hate vortex.