As box sets go, the Chailly is pretty good overall. The audio of the set is above average. Some of the performances are quite good.
Don't get put off by folks here. When you've heard hundreds of recordings of every symphony and have developed personal tastes for the exact way each symphony must be played in order to be satisfactory, it is impossible to find one set that has a great performance of every symphony.
I think people here would criticize Mahler himself conducting his own symphonies, because he'd have the inner movements of the 6th in an order different than they preferred, or too many or too few hammer blows, or not enough organ in the 2nd, or not enough cowbell, or too fast or too slow, with tempi too fluid or too rigid, or whatever.
Since it has good audio and reasonably priced, the Chailly set is a good way to get going. All of the performances are adequate, some are very good. Compare the performances to other recordings that you pick up. If you get the Mahler bug, as many do, you'll end up trying a lot of different recordings and eventually deciding what sounds "right" to you. Most people here have a bunch of full sets and hundreds of individual recordings, and they've decided on the 2 or 3 recordings of each symphony that they like best. But a year later, many will have a different favorite. If you drop in at that other Mahler discussion group, you'd find that their choices would be quite different.