most people report slugs to shoot about 6" high and to the right (iirc) on the jm at 50 yards. i just ordered some slugs yesterday so i will have to see.
as for reliability, i hear from the majority of people that they are very reliable, but... i have heard of more complaints about them on different forums than some other (more expensive) sasg's. but you know how it goes with posting complaints on forums, you will always hear more complaints on a forum than praises. i dont have enough rounds through mine to effectively comment from personal experience, but so far so good.