If performed correctly, without human error, it should kill inmates quickly and painlessly. Yet each of the four main steps involved — securing IV access and then injecting each of the three drugs — has raised medical concerns. Starting an IV can be very difficult on a person who is obese, nervous or cold or who has a history of IV drug use, any one of which is not an unlikely complication in an inmate about to killed. Sodium pentothal is not packaged in solution, meaning the executioners must mix the powder with water just before killing, a somewhat delicate thing to do. The pancuronium bromide, or paralytic, prevents observers from determining if the inmate is properly anesthetized, since he can’t speak or move. Potassium chloride, which stops the heart, creates a burning sensation in the veins and might cause excruciating pain if the inmate is not properly anesthetized. (These concerns, and others, appear in the 1953 British report.)

https://web.archive.org/web/20170203051825/http://www.nytimes.com/2007/02/11/magazine/11injection.t.html

Anaesthesia during lethal injection is essential to minimise suffering and to maintain public acceptance of the practice. Lethal injection is usually done by sequential administration of thiopental, pancuronium, and potassium chloride. Protocol information from Texas and Virginia showed that executioners had no anaesthesia training, drugs were administered remotely with no monitoring for anaesthesia, data were not recorded and no peer-review was done. Toxicology reports from Arizona, Georgia, North Carolina, and South Carolina showed that post-mortem concentrations of thiopental in the blood were lower than that required for surgery in 43 of 49 executed inmates (88%); 21 (43%) inmates had concentrations consistent with awareness. Methods of lethal injection anaesthesia are flawed and some inmates might experience awareness and suffering during execution.

https://web.archive.org/web/20130729013249/http://www.thelancet.com/journals/lancet/article/PIIS0140-6736%2805%2966377-5/abstract

I mean, it's all subjective. The electric chair is up there, for sure. I do think I'd rather be gassed. But the thought of being paralyzed, unable to move, while my veins are burning? That's terrifying. My main question is: why the paralytic? The barbiturates are supposed to put you under, and the potassium is what kills you. The answer is the comfort of the executioners and witnesses.