I visited the Mausoleum ten years ago. It is solemn but a bit creepy experience. After waiting in a long queue you enter the underground crypt and pass slowly in pitch darkness round the opened coffin protected by bulletproof glass and illuminated by bleak light. After that you emerge on the surface again and, squinting your eyes, find yourself strolling along the Kremlin wall necropolis with tombs of Soviet leaders - Stalin, Brezhnev, Andropov etc. There are also mass graves of revolutionary fighters and many urns with ashes of famous people: writer Maxim Gorky, aircraft test pilot Valery Chkalov, the father of the Soviet atomic bomb Igor Kurchatov, rocket engineer Sergei Korolev, the first cosmonaut Yuri Gagarin and marshal of the Soviet Union Georgy Zhukov, to name just a few.
Some tips for prospective visitors. It is not allowed to stop and stare or even talk while inside, and you'll be warned if you don't comply with the rules. Also guards at the entrance will stop you if you're carrying any bags, so you should leave your stuff somewhere, for example in the paid cloakroom, before joining the queue.