Disclaimer: This is an incomplete attempt at analyzing Hickey. There's just too many variables to actually attempt and say anything worthwhile. I don't think it's fair to draw too many assumptions in cases like this. With that being said, here we go.
July 2004 - November 2006 Houston Astros PC
November 2006 - Present Tampa Bay Rays PC
1998-2004 Triple-A New Orleans PC
1996-1998 Double-A Jackson PC
Let's focus on his major league jobs. First up, whether the Astros pitching staff improved after Hickey took over in July of 2004. Using FanGraphs split tool, I pulled the notable metrics for each of the months during that season, and as you'll see below, the results are mixed. July was the second worst performance from a FIP standpoint, and August was the absolute worst, yet in September/October the performances picked up to post the best month of the year.
The staff FIP that season was 4.07, good for fourth best in the majors and second best in the National League. In 2005, the Astros posted a staff FIP of 3.84, second best in the majors behind the Marlins. It's hard to decipher whether this is an improvement based on Hickey's teachings, or whether the staff's talent level increased. Consider the rotations, game started in parentheses:
2004 - Oswalt (35), Clemens (33), Redding (17), Munro (19), Miller (15), Pettitte (15), Backe (9), Hernandez (9), Duckworth (6), Oliver (2), Griffiths (1), Fernandez (1).
2005 - Oswalt (35), Pettite (33), Clemens (32), Backe (25), Rodriguez (22), Astacio (14), Duckworth (2)
More consistency from the guys you want pitching. The Astros used 23 pitches in 2004 and only 17 in 2005. Returnees: Pettitte, Lidge, Backe, Duckworth, Harville, Qualls, Wheeler, Gallo, Clemens, Oswalt, and Springer. Let's look at how they did:
2005: 1,179.3 IP, 0.99 HR/9, 3.18 BB/9, 7.51 K/9
2004: 896.8 IP, 1.13 HR/9, 3.15 BB/9, 7.75 K/9
Not much of a difference there.
The Astros 2006 FIP was 4.27, again fourth in the majors. Roger Clemens would only make 19 starts, Rodriguez 24, Buchholz 19, Nieve 11, Hirsh 9, Backe 8, Sampson 3, and Albers 2. Despite that, you have to concede that the Astros staff was pretty good.
Hickey moved to the Rays for the 2007 season and saw the Rays post a 4.7 FIP (4.38 from the rotation). Meanwhile, the Astros staff finished with a 4.73 FIP. Hickey's guidance saw Kazmir post a sub-3.5 FIP, Shields a 3.86 FIP, Sonnanstine a 4.26 FIP, and so on. Some pitchers, like Edwin Jackson, Ruddy Lugo, and Shawn Camp, took steps back. A discernable reason is lacking, Jackson threw more sliders and changes than ever before in his career; Lugo's velocity was slightly down and became more reliant on his breaking pitch; Camp threw more fastballs than in previous seasons.
Last season, the Rays FIP improved to 4.22, the Astros stayed in the bottom third of the league. So far this year, the Rays have a 4.79 FIP. If you look at the results only, this is the first year that a Hickey-coached team has been this bad. This is also the first year where a Hickey coach teamed has had its ace fall apart at the seams and two other members of the rotation become ridiculously ineffective.
At this point, you have to pause. The reasons for success and or failure cannot be attributed directly to Hickey without more intimate knowledge about his instructions. Take Roy Oswalt, since Hickey has left Oswalt has begun throwing his slider more and curve/fastball less. Oswalt has also seen his FIP climb each season since. Is that because Hickey understood Oswalt's strengths better than anyone else or is it merely coincidence?
Coaching is one of the few aspects where we almost have to defer entirely to the front office. There's simply too much statistical noise to make any definitive statements one way or the other.