Yes but they usually come out a bit 'one-toned'. You wont get the natural denim fibre look anymore. The best ones are the ones that go in the washing machine, much better coverage.
Well, I bought a colour described as denim from Wilko, but they ended up a bit too dark. To be honest, they looked like trousers from affar and I don't wear them now because they do look dyed!
Hmmm, if you don't mind them being thrown in the bin if it goes all wrong, go for it! Just think if you would wear them anyway! ;0)
Dylon is the best make, the best one is for use in the washing machine, you will also need to buy salt -- read the instructions first but yes, you will get an 'all over' look unless you try tie dyeing. Good luck !
You also need to bear in mind that the fabric of the zipper and the stitching / logos may take the dye differently - if you do this for fun and it turns out wearable you've won, but if they are beloved jeans I would just keep on wearing them as they are till they fade away altogether.