As others have replied, it is mainly the poor performance of the outer points that people complain about, but there is more to it than just that: the centre point is only better than the outer points in terms of its performance with vertical structure (eg. lines) - its AF performance with horizontal detail is just as bad as the outer points. It is the vast difference in the focus precision with vertical detail over horizontal detail that makes the centre AF point acceptable at all.
When Canon refer to the centre AF point on the 5D/5DII being a "cross type" they mean a "+" cross rather than an "X" cross (as they have in the 7D, for example). To understand the difference you have to realise that the AF sensors are really just linear sensors, sensitive to edge detail crossing the length of the sensor. If the edges are at an angle to the sensor then the AF is only sensitive to the component of the detail along its length - ie. the cosine of the angle between the detail and the AF axis.
In other words, the sensitivity of the AF is maximum with lines parallel to the AF axis (at 90deg to the linear sensor), and reduces as the angle between the lines and the AF axis increases, eventually falling to zero sensitivity when the lines are perpendicular to the AF axis.
That probably reads as overly complex, but it is easier to understand and demonstrate with the camera itself. Just select any of the single axis AF sensors, ie. the outer sensors, noting that each of the outer AF points is shown as a small rectangle in the viewfinder, not a square. Point the sensor at a scene with only a sharp black/white edge, such as a the edge of a white sheet of paper on a black background crossing the AF sensor. When the black/white edge is parallel to the short edge of the AF marker, the camera focusses with maximum accuracy and this falls off as the edge rotates relative to the sensor. When the edge is parallel to the long edge of the AF marker the camera cannot focus at all - the AF sensor has no sensitivity along that axis at all. Focus sensitivity is the cosine of the angle between the edge and the AF axis, maximum at zero, minimum at 90deg.
Obviously, with a cross-type sensor you never get into the situation where the AF sensor has no sensitivity to image detail. As the lines of detail approach angles with minimum sensitivity in one axis of the cross they also reach maximum sensitivity with the other axis of the cross. This makes the centre point on the 5D/5DII AF much more robust and reliable than the outer points but, again, there is more to its performance benefits than just reliability. It can also be more precise, especially with fast glass.
The central AF sensor is actually 3 AF sensors in one: one for each axis of the "+" cross, but also a third, high precision, AF sensor aligned with the "-" axis of the cross and sensitive only to vertical edges. This third AF component of the cross-type sensor becomes active with lenses which have an aperture of f/2.8 or faster. So only two AF sensors work at any one time. If the lens is slower than f/2.8 then both axes of the cross-type sensor work in low precision mode; with lenses faster than f/2.8 a high precision sensor sensitive to vertical detail kicks in, although sensitivity to horizontal detail remains at standard precision. The difference between these two is very significant and further enhances the performance of the central AF point over the outer points.
On one of their Lenswork publications, which I don't have at my fingertips at the moment, Canon claim the precision of their standard AF sensor is equal or better than the DoF, whilst their hight precision AF sensor is equal or better than one third of the DoF. On the face of it, that looks like the high precision AF sensor is 3x better than standard AF. However, this is the performance of the sensor at the working aperture of the AF itself. The standard AF sensors work at f/5.6 (this being the separation of the outer edges of sensitivity of the two optical ports that the AF "sees" through the lens), whilst the high precision AF sensor works at f/2.8. The depth of focus at f/2.8 is half the depth of focus at f/5.6, so the high precision AF sensor is actually 6x more precise than the standard AF sensors - and that is where the real benefit of the centre point comes in.
Consequently, if you are shooting at f/1.4, where the depth of focus is a quarter of that at f/5.6, the outer AF sensors can only get within 4x the depth of field - almost useless. However the depth of focus at f/1.4 is half that of the f/2.8 aperture of the high precision central AF sensor, so that can still achieve focus within 2/3 of the depth of field - 6x better and a critical advantage.
These comparisons are, of course, under optimal conditions with the edges of the detail perpendicular to the AF sensor. However the difference is so significant that even with detail which isn't perfectly aligned with the AF axes, the high precision sensor still dominates the performance of the central AF sensor in the 5D/5DII system. When only the standard precision sensors function the crossover between the vertical and horizontal axes occurs around 45 deg, where the precision of the AF has reduced to 70% of the optimum performance. When the high precision vertical sensor functions and the crossover between the two AF sensors occurs around 80 deg, so performance never falls below 98% of optimum for the standard AF sensor.
So the difference between the central AF and the outer points on the 5D/5DII is significant, not just in precision, but in reliability. Quite often, the outer points will hunt and fail to focus completely whilst under the same conditions the central AF point will work and still give better AF accuracy, especially with fast glass.
