Hey Joseph, I own the IS version and it is positively my favorite lens  
(and easily my most expensive). The answer to your question is another  
question: What sort of photography do you do? I photograph wildlife  
(primarily Whitetail Deer) and in that capacity it was worth the extra  
money. If you shoot primarily hand-held, in poor indoor lighting or dim  
forest lighting then it will be worth it. If you always use a tripod, use  
flash alot, and are most concerned with shooting outdoor sports in the  
middle of the day then no, in my opinion it would not be worth the  
signifigant price difference.
  I don't own/haven't used the non-IS so I can't compare the focusing  
speeds, but everything I have read says both are great lenses! Hope this  
helps!

Kelly

I wouldn't even consider the non-IS version - you'll need almost 8 times as
much light to be able to hand-hold a marginal shot (3 stops).

A friend of mine bought the non-IS version - I bought the IS version. Now he
borrows mine, and is thinking of upgrading his (and taking a loss).

Hope this helps!

check out the new 70-300 IS.  It gets good reviews and is under $600.
http://www.fredmiranda.com/reviews/showproduct.php?product=294

   Depends on what you shoot.  At 200mm, hand-held shots can become
difficult due to camera shake.  The IS will help drastically with these
shots when the shutter speed is less than 1/125s.  For sports (where this
lens is very popular), the IS is generally no help because the motion of the
subject demands 1/250-1/500s shutter speeds.  But most sports shooters still
buy the IS version if the weight and extra cost aren't a big factor
(monopods and paid-for equipment).  I've never heard them say there are any
differences between the IS and non-IS in focus speed, and since sports
shooters are demanding of autofocus then any speed difference must be very
small.

   Dave

And the third, the f/4, doesn't have stabilization, is a stop
slower, much lighter and much cheaper.

You might want to look here,
   http://www.photozone.de/8Reviews/index.html  
where e.g. the measured MTF50 shows a drop for the IS lens,
at 70mm a weaker border (1.6x crop!) resolution, and they claim
it's a bit more flare prone.

If you always shoot in conditions where you get 1/250s f/4[1]
(the effective focal length is 320mm @ crop 1.6) or use a monopod
for, uh, I guess 2 steps of stabilization[2] or use a tripod for
static objects, then the f/4 is your choice.

If you need to watch weight and space, think about the f/4.

If your budget is tight, the f/4 (don't forget,the tripod collar
is extra here).

If you want to use a 2x converter on a non EOS-1 body and retain
AF, the f/4 is contraindicated.

If you need a *single* stop less DOF or a *single* stop more speed,
the f/2.8 non-IS is your choice.

If you shoot subjects where a 'slow' 1/60s is acceptable and you
cannot use a monopod or tripod (you can really help the lens with
a monopod, too), the IS lens is nearly worth it's weight in gold
(about $30.000 :-)).

If you like panning horizontally, the IS can help a lot, too.

As above, if you shoot tripod only, you'll be probably better
of with the f/4.  If you use monopods extensively and you don't
fancy 320mm at 1/10s[3], you don't need the IS.  If you however want
1/40s or 1/60s@320mm[4] without a monopod, the IS is for you.
Same if your tripod is on a bridge and a truck passes you ...
or the wind is strong and the tripod's not 100% vibration
free.

-Wolfgang

[1] f/4, 1/250s
   ISO  100: bright overcast days
   ISO  400: dark overcast days (London type)
   ISO 3200: indoors at day, circusses, brightly lit professional sports

[2] f/4, 1/60s:
   ISO  100: dark overcast days (London type)
   ISO  800: indoors at day, circusses, brightly lit professional sports
   ISO 3200: indoors at night, bonfire lit subjects

[3] f/2.8, 1/10s:
   ISO  100: brigtly lit indoors at night
   ISO  400: Subjects lit by a single candle, flootlit buildings
   ISO 1600: typical lit night street scenes
   ISO 3200: dark night street scenes

[4] f/2.8, 1/60s:
   ISO  400: indoors at day, circusses, brightly lit professional sports
   ISO 1600: indoors at night, bonfire lit subjects
   ISO 3200: Subjects lit by a single candle, flootlit buildings

Trust me, the price difference is justifiable.  Even if you don't plan on
ever taking a photo without the camera/lens on a tripod, it can be of help.
And if you try to hand hold that puppy, you'll NEED the IS.
Long ago, I swore I'd never get another long lens without IS, it's that
valuable.