I have heard and read people disparaging the Lancaster as compared with the B-17 due to the former's comparatively low operational ceiling, but its important keep in mind that these two aircraft were designed for (and carried out) different roles.
The B-17 was designed fly at a high altitude and to indiscriminately shower their targets crap-loads of bombs. It carried out that role very well, but during WWII at least, that is pretty much all it ever did.
However, the Lancaster was designed as a medium to low altitude precision bomber, to strike difficult to hit targets both in daylight and at night. It was used in the precision raid on Peenemunde (Rocket Island), the German experimental weapons facility which produced the V1 and V2. It was also very versatile; modified to carry Barnes-Wallace's "bouncing bombs" in the famous Dambusters raid on the Ruhr Valley dams, a mission that required it to carry out its bombing runs at 60ft!!!
The Lancaster's long, single bomb bay was also ideal for carrying the giant "earthquake bombs" Tallboy and Grand Slam. They used the former to good effect when they sunk the German Battleship Tirpitz in 1944. Yet when called upon, it was still able to do a similar role to the B-17 when it took part in the strategic bombing offensive, such as the massed attacks on Hamburg in 1943 and Dresden in 1945
The Lancaster was RAF's premier strike aircraft from 1942-45.