Been on my bucket list for 20 years. Approached by human power from Petropavlovsk-Kamchatsky (mountain bikes for 600km and backpacking for 45km.) with SP member Laverna. Waited two days for weather at mountaineers' hut at the col (3200m) between Klyuchevskaya and Kamen volcanoes. Attempted on the third day, but clouds rolled in. On the fourth day, went up solo and dodged cannonballs. Retreated 15m from the crater rim because I became dizzy from the fumes. See our trip report:
Rather dull summit, enormous prominence and objectively dangerous due to falling rocks (they fall randomly as the volcano shakes without human or weather intervention at all). However the views were rewarding. The GPS showed 4850 and we were extremely lucky to see a small new cone spitting lava!
Started early to avoid stonefall danger as more as possible. 3 A.M. from the BC on 2800 and reached the summit at 8 A.M. The weather was perfect. The GPS showed 4780 at the brim of the crater but the opposite (southern) side of it looked one hundred meters higher but was unaccessible from our side. Easy snow climbing but must be very attentive because of falling stones. All necessary equipment was crampons, ski-poles and helmets
Petr Novak and I climbed up Klyuchevskaja with crampoons with skis on our backpacks. I had to finish due to health troubles (also bad snow conditions) at approximately 3500 m. Petr reached the summit from BC (2070 m) in six hours and he spent two hours on the north side of crater to find in fog the highest point using GPS. Then, he descended on skis directly to the north - very icy and stony slope...
Very nice weather, good conditions, Start early - snow's getting soft in afternoon (watch out for loose rocks!)
Crater emitting sulphuric smoke - not very healthy or nice to inhale...