That could be a long answer - so I'll try for the concise version,
with the option for you to ask more specifics as required..
The one keyword is written in a reply above : versatility.
I'm a narrow, technical board guy from way back, well used to taking the good (top end potential) with the bad (compromise on just about everything else), so it's really positive to have some development like iS that (in general) maintains the performance of more demanding designs but reduces the % of compromise on all the other factors.
Expand the sweetspot.
It's fair to say across the 2006 slalom range the iS really (for most circumstances) went a step ahead of most of the other options out there ~ and the 2007 iS's are a definite second generation improvement. (Not to imply other brands are off the pace, but having shown the way in 2006, obviously there is some advantage to being a bit further down the development road - and let the results and market feedback speak for that).
With iS range being first tested against the (well proven) Sonics, there was obviously some scepticism (more from the die hards) that a wider tailed design would not (could not !) go or turn as well as the benchmark/s (Sonics 100/95 etc). Well, making the long story short, the 105 wasn't nearly as "embarressed" in head to head with the 100/95 as expected, and in fact even early showed some additional promise in certain areas (range/accel/lulls/windward/carrying power thru jibes) - and with some minor rider technique retuning(most notably trim at high speed across chop), the "match" to S95 started to turn to "advantage" iS105.
(sidebar here : I guess the ~100Lt / 6.5 to 7.5m sweetspot range is also about the ideal "size" for broadranged open water slalom racing/blasting/drag racing with most experienced riders. With moderate input, a really good and well tuned ~100Lt /7.5m slalom can be planing and very effectively (open water) fast in quite light winds, while still (~6.5m) be great fun & on the pace when most open water conditions start to get too choppy or lumpy for real fast boards to be a general advantage. For most, it's the workhorse, one size fits all size - so having a design in that size that delivers enhanced ride (hi speed trim) and range (versatility) with minimal (or no) compromise in other factors over other boards in that range is a pretty compelling recipe.)
In testing out some smaller, wider tailed protos for the (never built) iS90, the benefit of the range and trim at high speed became really obvious, but compromise loomed in the high speed control/jibe area. When Per told me he hit on a super proto for the 105 replacement with a wider tail (so enhanced iS planshape !) that jibed much better and had same or better light wind speed than 105 (in critical terms), well I was a bit "yeah, I'd like to see that.." but he was (as usual) right on the money and the thing was for sure a better, more reactive and easier (almost freeridish) ride than the 105. Enter the new 101.
For me, the 101 hits the peak of the curve ; the smallers IS's can still be faster (in the right but increasing narrow band of conditions) and the larger iS's are for sure also deceptively fast - but get overshaddowed and harder work once sailed into the 101's very wide & accessible sweetspot.
Nope, one size doesn't fit all, (rider or occasion) but (at least for me) if there had to be a one size choice it would for sure be 1-0-one.
Cheers ~ Ian
(ok, it went a bit long..)