Bought a 25 lb kettlebell and picked up the book, "Kettlebells for Dummies" and make great progress. I leaned out nicely and lost some body fat particularly in the torso area. I'm hoping to save my pennies and purchase a 35 lb one in the very near future as I'm starting to outgrow my 25-pounder on certain lifts. Love doing the swing, clean and presses, squats and turkish get-ups with them. I never got bored when working with them...fun workout! Hope that helps.
ETA: I was working out with free weights for awhile so hence the reason for purchasing a heavier one. If I was a relative beginner, I would've started out with a 10-15 lb one personally. You want to challenge yourself but not to the point where you cannot perform a move like a press (above your head) because it's too heavy. Form over ego IMO.