The following studies found that circumcision does have a detrimental impact on penis sensitivity.
From a 2013 study by Bronselaer et al.:
The analysis sample consisted of 1059 uncircumcised and 310 circumcised men.
For the glans penis, circumcised men reported decreased sexual pleasure and lower orgasm intensity. They also stated more effort was required to achieve orgasm, and a higher percentage of them experienced unusual sensations (burning, prickling, itching, or tingling and numbness of the glans penis).
For the penile shaft a higher percentage of circumcised men described discomfort and pain, numbness and unusual sensations.
In comparison to men circumcised before puberty, men circumcised during adolescence or later indicated less sexual pleasure at the glans penis, and a higher percentage of them reported discomfort or pain and unusual sensations at the penile shaft.
From a 2008 study by Yang et al.:
There is a statistic difference in the glans penis vibration perception threshold between normal men and patients with simple redundant prepuce. The glans penis perception sensitivity decreases after circumcision.
From a 2007 study by Sorrells et al.:
The glans of the circumcised penis is less sensitive to fine touch than the glans of the uncircumcised penis. The transitional region from the external to the internal prepuce is the most sensitive region of the uncircumcised penis and more sensitive than the most sensitive region of the circumcised penis. Circumcision ablates the most sensitive parts of the penis.
From a 2006 study by Kim et al.:
However, circumcised men were more than three times more likely to report less enjoyable sex lives after circumcision than better sex lives (28 vs eight men). While decreased sensation was the most frequently cited reason (21 of 28 men) for a less enjoyable sex life, complaints about the physical effects of circumcision on their penises and consequent adverse effects on sex life were also prominent (13 of 28; multiple complaints were separately counted).
In summary, we studied the effects of circumcision on sexuality. There were no differences in sexual drive, erection and ejaculation, but circumcised men reported decreased masturbatory pleasure and sexual enjoyment. We conclude that adult circumcision adversely affects sexual function in a significant number of men, possibly because of loss of nerve endings.
From a 2001 study by Boyle et al.:
A survey of the 35 female and 42 gay sexual partners of circumcised and genitally intact men, and a separate survey of 53 circumcised and 30 genitally intact men themselves, indicated that circumcised men experienced significantly reduced sexual sensation along with associated long-lasting negative emotional consequences.