Tactically, tanks could be decisive in the breakthrough role when used effectively as at Cambrai (namely in quantity, density, and in coordination with other arms). They were immune to rifle and machine gun fire and as no dedicated antitank artillery had yet been developed they proved frightening and difficult for the Germans to destroy. In addition, they could cut paths through barbed wire for following infantry and the British Mark series tanks in particular had virtually unlimited trench crossing capability.

However, tanks could still be damaged or destroyed by existing field artillery, hand grenades, and land mines, as well as the antitank rifles developed late in the war by the Germans. Their advance (as with other service arms) could be slowed or brought to a halt by heavy mud or mechanical failure. They required close coordination with infantry, artillery, and cavalry to be effective. Nevertheless, they were a valuable breakthrough and morale weapon, if not enormously practically superior to German strormtroopers utilized in their 1918 offensives.

Operationally and strategically, tanks were problematic. Although lighter, faster tanks were developed (ie, the Whippet) WW1 tanks as a whole were heavy, logistic and maintenaince intensive, slow, and mechanically unreliable. This meant that large numbers had to be used to ensure enough working vehicles were available, and they could not be easily redeployed as trains would generally have to be used for trips of any length. This also meant that a tank offensive in the making could be spotted long before it occurred--though admittedly this problem was not at all unique to tank offensives in WW1.

Unreliability and low speed also limited the potential of tanks to exploit the breakthroughs they created, meaning that this job usually had to be left to cavalry. In addition, while operational tanks could generally navigate No Man's Land effectively, the major problem of reinforcing a breakthrough once created remained. Allied infantry, cavalry, and (especially) artillery bogged down in the heavy mud, while fresh enemy reinforcements could be quickly brought in by rail, and the concept of tracked personnel carriers had not yet come to fruition.

EDIT: On crew: forgot to mention the absolutely terrible situation of WW1 tank crews. Between the engines and the tank's own weapons firing, the inside of tank rapidly became very hot and noisy, and crew members were sometimes killed or wounded by spalling from non-penetrating hits. In addition, because of the newness of the technology and the relative lack of viewports, being inside a moving tank could be disorienting for the crew. These factors reduced crew (and thus tank) effectiveness.

TL:DR: Tanks made breakthroughs easier, but didn't solve the other intrinisic problems of WW1 offensives.