Jen-Ho Tseng is considered one of the better international prospects in this year's July 2 international signing class. An 18 year old (turns 19 in October) right-handed Taiwanese pitcher, he was on the WBC roster for Taiwan and made an appearance. According to scouting reports, he has a fastball that has gotten up to 95 MPH, an okay slider, and a good curve, that has also been described as a knuckle curve. There seem to be quite a few good videos of him online, and that is where much of my information about Tseng comes from. Many of the hyperlinks in this post are videos on Tseng (as I tried to give credit to the ones I gleaned information from).
From field level, he certainly looks like he has good enough size. It doesn't look like he will fill out much more though (David, @Yankeesource, notes that he is already physically mature and thinks the stuff is big league stuff, but he doesn't really have projection). He's 6-1, which is okay height for a starter, but probably below MLB average.
The Hiroki Kuroda comps are stupid, especially since he doesn't throw a splitter, and Kuroda is one of the best pitchers in baseball (also, Tseng might throw harder). The delivery seems to be somewhat similar, but Tseng's pause seems more dramatic. There seems to be some deception as he turns his back somewhat to the hitter for a split second. A good sideview of his mechanics shows us that he isn't really doing anything we haven't seen before.
His fastball isn't flat or straight, moving somewhat like a two seamer. He evidently has a tendency to yank it quite a bit, but seems to work both sides of the plate with it (two seamers are usually thrown more arm side than 4-seamers). However, in centerfield camera shots, it is pretty clear that it is more of a 4-seamer, just with some movement and an ability to get on top of it well. It does look like he has a separate sinker.
Everywhere you look when it comes to reports on Tseng mentions the same 95 MPH he hit in an event in 2012. While that is impressive and makes him a big league prospect, I am much more interested in where he sits normally than what he hit in one outing. There are reports of him throwing 94 MPH back in 2011 as well and we see him throwing 93 MPH here, so in all likelihood, we are talking about a big league fastball. Ben Badler's report has him throwing 89-92 MPH on average. in 2012, but noting a velocity drop in outings in 2013. In fact, he looked pretty bad in the WBC, both by command and velocity. Whether this is arm problems or just problems with the ball (like we saw with Kenta Maeda) is impossible to tell.
Badler also notes that his best pitch is a changeup, something the Wikipedia pages have him throwing, but something the MLB.com scouting report omits completely (and admittedly, something I didn't see a lot in videos) . You can see why the change is rated highly, as it looks somewhat like the fastball before breaking arm side quite a bit. Control of it seems to be an issue, but it could be a plus pitch.
The curve at times looks like a knucklecurve (as opposed to a more traditional curve or a 12-6 curve), with slow break to the glove side. At other times, it looks more like a big 12-6, clocking in the high 70s according to the radar gun (but it looks slower). From what I saw, this was his best off-speed pitch and he could throw it for strikes. His slider is also in the high 70s and up to the low 80s, and is the more of the baby slider you usually see in the Far East, a slider more focused on going down than sweeping, thrown usually for strikes, and one that is thrown to both lefties and righties.
What his velocity actually is seems to be a question, and makes him hard to judge. If it is the good velocities that we saw in the 2012 videos, he is a good MLB prospect. If it is what we saw in the WBC, he is pretty fringy.
How he has been used in the past is a question that is very important and would be worth researching for interested teams. Amateur baseball across the glove is notorious for high pitch counts, which isn't always good on a young arm. Could a high usage have caught up to him and caused a terrible spring? Could it have lead to a permanent loss in velocity? How the pitcher has been used as far as pitch counts and days of rest goes would be something teams would probably have at least a general idea about, more than I have to go on. I think a physical for Tseng would be very interesting and important for how much money he should get as a bonus.
Making comparisons are difficult, because most Asian pitchers have approaches, deliveries, and pitches (for example, his slider is much different than most sliders you see in the States, and he seems to use his curveball the way you see in the NPB, as a get me over strike pitch to mix in like a regular pitch, while most MLB pitchers use it relatively sparingly and as a put away pitch) you usually don't see in the United States outside of pitchers originally from Asia. This makes most comps impossible or racially lazy (like the Kuroda comparison). While I am not crazy about comps, I like comps when it comes to a pitcher's stuff, but I think for Tseng, it is best to just not make a comp. What his ceiling is depends on fastball velocity. He has other pitches that look like big league pitches and it looks like he can command, so if the fastball velocity is there, he could be a very good prospect.