There are 3 main types of shifters.  Shimano's Rapid Fire plus, Sram's 
Grip shift and Top Mount thumb shifters from various companies.

1) Rapid Fire Plus
-The system has a built-in brake lever, so if you buy the shifter, you 
 must use Shimano's brake levers (you can use a shifter perch on the
 higher-priced models).  This will be changed in 1996.
-A shift to the smaller ring (front and back) is done by a pull of the 
 index finger.  A shift to the larger ring/cog is done by pushing the 
 button with your thumb.
-Advantages:  Most claim that the RF+ keep their hands in fairly natural 
 position.  They don't have to move around to shift/brake.  The shifting 
 is very smooth, especially when matched with a rear derailleur made in 
 the same year.  Some feel that the optical display is very useful.  
-The disadvantages:  Heavier than topmounts/grip shifts.  More 
 expensive.  You can't buy your own brake lever (shifter perches are 
 exceptions).  Some feel that using Shimano parts is a "shame".  Some feel 
 that the optical displays are crap.  Downshifting in the back is limited
 to about 3 gears and upshifting is only one gear per push.

To this, some added:
John Stevenson [johnstev@world.net]
Only XT will have separate RF+ units for 1996.  All other groups are still
"integrated".  Perches are available from various vendors for both early RF+
(without 'Optical Gear Display' windows) and for OGD RF+ units.

2) Grip Shift
-There are many designs, the most popular one is made by SRAM.
-These are shifter units made to mimic the "twisting throttle" motion of 
 a motorcycle.  Comes in shifter units only, you must supply your own 
 brake levers.  In 1996, they will introduce their own brake lever/shifter 
 units and also they will make shifters that are only compatible with 
 their own rear derailleurs.
-To shift to larger ring/cog, roll your wrist forward.  To shift to a 
 smaller ring/cog, roll your wrist backward.
-Advantages: Cheap.  Light.  Simple.  Natural hand position.  Easy for some 
 to use because of their motorcycling background.  Simple to overhaul.  
 Great customer service.  Favored by many new riders and some experienced 
 users.  Can shift through all the gears in one twist.
-Disadvantages: Unwanted shifts can occur when going over bumpy trails.  
 Some people don't like the hand rolling motion.  Cable routing can cause 
 some novice mechanics trouble.  Cannot brake and shift at the same time.  
 Might be troublesome if used with conjunction with Shimano Light Action 
 rear derailleurs.  Cannot fit on some multi-position handlebars without 
 cutting the barrels (voids the warranty).

Some added:
John Stevenson [johnstev@world.net]
SRAM's Grip Shift is a shifter unit which turns only a portion of the gips
- roughly one-third, depending on how long you cut your grips.  Campagnolo
tried and abandoned a full grip shifter, SunTour produce a 'partial grip'
shifter like Grip Shift, as do Sachs.

SRAM's 1996 range includes one combined shift/brake unit, aimed at low end
OEM use.  The top end SRT 900 shifter is claimed to be only compatible with
the 900 rear derailleur.  The mountain bike community usually treats such
claims from manufacturers with scepticism.

[With regard to Grip Shift problems]
Massive problems in wet conditions: poor sealing means that mud rapidly
erodes the internals, leading to mushy shifting; can be very dificult to
grip when wet and muddy; 1995 models with 'Fastest Front Shifting' require
considerable hand force to shift on some bikes - very dependent on quality
of cable set up and smoothness of routing.


3) Top Mounts
-Separate unit from the brake lever.  
-To shift to a larger ring/cog, you push the lever forward, and back to
 shift to a smaller ring and cog.
-Advantages: Simple, no real moving parts.  These shifters last for a very 
 long time.  Equipped with friction mode so that you can ride with a mal-
 adjusted derailleur.  Light and usually cheap (if you can find them).  
 Favorites of many more experienced riders.  
-Disadvantages: Very hard to find.  Shimano discontinued their shifters in 
 1994 and Suntour is not in business in North America.  You need to move 
 your thumb out of its "natural" position to shift.  Some feel that moving
 their thumb after a long ride is very tiring.  Not very "trick".

Final words:  With the amount of research going into shifters, all 3 (top 
mounts, RF+, GS) shifters are great.  They all shift very well.  They all 
have their own unique strong points and weak points.  It is impossible for 
one to say that one type of shifter is superior to another type.  The 
most important factor in deciding what to buy is personal preference.  Go 
with what you like, you will get great shifting if you put in enough money.