Fact 2

Always test a club's swing weight before you remove the old grip, grip weights can vary quite a lot, changing the swing weight and the feel of the club once fitted. If you have golf clubs with particularly high or low swing weights the wrong grip could render them un-playable. By knowing the original swing weight you can maintain a golf clubs characteristic. Unless you know what you're doing and have the right equipment, renewing your grips is not a DIY job.

Fact 3
If your grips are too large for your hands, your wrist movement may become restricted, resulting in a pushed or sliced shot. The golfer may then, subconsciously, grip further down the club to find a comfortable point where the grip is smaller, resulting in a reduced swing speed and a loss of 2 to 3 swing-weights.
If your grips are too small for your hands you probably have a tendency to turn your hands over through impact, causing pulled and hooked shots. The grips may twist in your hands causing you to grip too tightly or to re-grip the club during the swing, or you may lose your grip at the top of the swing losing contact and control of the club. 

Fact 4
Physical problems, such as Tennis Elbow and Arthritis can be a problem for some; larger and softer grips can take a lot of the discomfort out of the game.

Fact 5
Hard and cord type grips have long been the preference of the pro and the better player; they work much better in wet weather and humid conditions, these come as standard on most golf clubs. Unfortunately many amateurs would benefit from the comfort and tackiness of a softer grip.

Fact 6
A good glove does not compensate for worn-out grips. You need good grips; if cash is an issue spend your glove money on grips and play without a glove. There are 1000's of single handicap golfers that don't use gloves, I'd be amazed if there were more than a handful with shiny grips.

Fact 7
I would strongly recommend the same style and size of grips for both woods and irons in your bags. A lot of golfers buy their golf clubs from different manufacturers, or buy used golf clubs. There's nothing wrong with this, but it's very important to maintain a consistent feel throughout the set, re-grip all of your golf clubs with the same grip model and size.