The primary advantage to the T-tail design (and the cruciform design) is a greater elevator moment and an expanded CG range.
A t-tail configuration in some cases removes the horizontal stab from airlfow interference of the wing, though not completely.
The t-tail is disadvantaged in being unable to provide an induced download from downwash from the wing.
T-tails typically suffer from deep-stall considerations separate from swept wing or other issues; the most obvious being a blanketing effect on the horizontal stab by the main airfoil (wing) during the stall, and a loss of pitching authority. Additionally, as the t-tail doesn't rely on a download from the main airfoil during normal operations, the introduction of download during high angle of attack operations may drive the stall deeper, or make it unrecoverable. The learjet is a good case in point.
Someone had mentioned the C-130; while I'd disagree that it's antiquated (it's the longest production run of any aircraft ever built, and is still being built), there was no need for a t-tail. That airplane has a different susceptability, which is the fin stall; a type of aerodynamic rudder reversal at high angles of attack on the rudder; high sideslip angles. Elevator authority is never a problem, the airplane has good stall characertistics, and it has a very large CG range. It also utilizes airflow fromthe inboard engines, and a download in most cases on the vertical stab.
On most light aircraft, the T-tail is entirely cosmetic. On several airplanes, the tail was moved higher to boost sales, not for reasons of aerodynamic purity.
The t-tail does tend to enhance spin entry and recovery authority. In many cases, greater rudder authority exists due to lack of interference by the horizontal stab. In designs with neutral inertia, this is important. In wing loaded aircraft, the elevator is the primary recovery instrument from a spin, and the placement of the elevator atop the tail enhances this in some designs, and can be detrimental in others. (On fuselage loaded aircraft, of course, the aileron is the primary spin recovery control, and the placement of the horizontal stab is of little consequence).
T-tails tend to require increased armature and rigging to effect control. The frequent use of jackscrew assemblies, additional bellcranks, runs of control rod or cable, etc, leave more vulnerability in the system, increase weight, and require heavier materials to retain strength with the additional arm and moment imposed by loads atop the vertical stab.
In some cases, t-tails were put there because there was simply no other place to put them. The 0V-10 is such an example. (although that design is a cross-tail, rather than a t, but is still mounted high on the vertical stabs).
Finally, you must look at the needs of each design on an individual basis. There is no blanket rule for the use of the t-tail. There are a great many variables, and each design is unique. It is impossible to state that the t-tail is good or bad; one must stick to a particular design to make such observations. In many cases, the use of the t-tail may be the best compromise for that particular design. Remember that any aircraft design is nothing more than a series of interrelated factors, and compromises. Many times, features of a design are not used because they are the best feature of a superior feature, but a good compromise.
Then again, it's hard to pay for a compromise unless it sells. Often the design features do nothing more than sell the airplane. Such is the case with a lot of T-tails.
