I agree with Handgunner in that its not much more than a 'hot' 9mm. Case length 2.5mm longer than the 'standard' 9x19 i.e. 9mm luger etc.

Here's more from an article by Massad Ayoob :
the .356 TSW round that Smith & Wesson and distributor Lew Horton debuted in 1995. It failed at the time because it was sold as an IPSC combo and was unnecessarily hot for that application. However, with the Cor-Bon 124 grain jacketed hollowpoint at a true 1,450 feet per second, the .356 now equaled the best .357 SIG and .357 Magnum of identical ballistics!

The .356 TSW (formerly: "Team Smith & Wesson"; now, more wisely: "Tactical Smith & Wesson") has an advantage over the .357 SIG. The SIG is a bottleneck round, a .40 S&W casing necked down to 9mm. The .356 TSW is a straight 9mm casing (9X21.5, for those who like details) and, thus, more rounds can fit into a magazine.

The only 16-shot .357 SIG is the Glock 31. It can reach full capacity using .40 caliber mags, but your customer must find grandfathered Glock 22 (.40 caliber) hi-cap magazines, which were only made for five years before the "no more than 10-round" ban. With the .356 TSW, your customer has ready and affordable access to millions of Smith & Wesson 9mm magazines made for a quarter century. They are still legal and will work fine with the TSW round.

The .356 TSW didn't sell the first time around because it wasn't marketed to where it had the most potential. Smith & Wesson won't make the same mistake twice. Mate this gun with 124 grain Cor-Bon hollowpoints, and you've got a winner. Pete Pi at Cor-Bon tells me he hasn't produced the round in years, but the last fresh stocks are available to you from one distributor who had faith and stocked up.