The process is not unpleasant but a bit strange feeling. In essence, what you have is similar to a bruise going on.
Unless you use too much vacuum, you won't experience bruising. An, of course, it feels "strange" the first time. Most of the sensation comes from skin stretching as the penis enlarges ... a bit like the old "indian burn" from someone grabbing your forearm with both hands and twisting in opposite directions. A maximum of 10 to 15" (260-380 mm) Hg (mercury column) is recommended.
Your
penis feels warm and numb it is like having a stuffy head feeling in your junk. When your nose is congested.
Your penis may feel warm but it won't get "numb" unless you pump too long. Sessions 10 to 15-minutes long are recommended.
The penis is slightly larger than normal but usually not significantly so.
My personal experience, which is not atypical, is a 25-30% increase in girth and 5-10% increase in length. Most of the increase will last an hour or two. Persistence pays off - doing 2 to 3 sessions a week, also recommended, will eventually make the increases permanent. When I started over 20 years ago, I was barely 6" long x 4.5" girth ... I'm now at a bit over 7" long and a bit over 6" girth ... and this is many weeks after temporary session results fade.
Sex or jerking off is also. A different and less enjoyable experience than without a pump.