I've never seen a Mercury capsule, but the most striking thing to me is its small size [1]. I know that the flight duration was quite short (longest was 22hrs) but the thing is tiny and there is basically no extra interior space.
Gemini was slightly bigger [2], but I'm always amazed that the Gemini 7 crew spent fourteen days in a space about as large as the front seats of a compact car.
Gemini was actually more cramped than Mercury. It had 160% the habitable volume of Mercury, but twice the crewmembers.
Gemini was known as the Gusmobile, because Gus Grissom basically lived at McDonnell during its development and tailored the design to his own preferences. And really tailored. Because Gus Grissom was the shortest astronaut at 5'5" (1.65 meters), Gemini turned out to be uncomfortable for everyone else.
Gemini was slightly bigger [2], but I'm always amazed that the Gemini 7 crew spent fourteen days in a space about as large as the front seats of a compact car.
Exceptional people.
[1] http://upload.wikimedia.org/wikipedia/commons/1/14/Mercury-s...
[2] http://upload.wikimedia.org/wikipedia/commons/b/bd/Gemini_sp...