A properly designed and installed shift kit can increase driving enjoyment and performance. It can also help to reduce transmission wear in an otherwise sound tranny- BUT it's NOT a cure for a worn out or broken tranny.
Some shift kits have different "levels", like RV/towing, street/strip or full competition. Whatever level you may choose, be SURE you stick to just the mods needed for that level- DO NOT "mix and match" mods or parts between the levels unless instructed to do so!
Some shift kits allow the gear manually selected to be held until the shifter is used to select the next gear, and will allow the tranny to be downshifted at any RPM- something that you should be aware of to avoid possible damage by over revving or loss of control by the rear tires locking up under engine braking.