"Brilliant Classics"
I would disagree in part with the first-posted response to your question. Brilliant Classics "big boxes" generally include a combination of recent recordings of lesser-known musicians or groups, mixed with older recordings of well known performers that have been licensed from the original recording company.
In the following examples, I've highlighted the performers who are more broadly known in the classical music world:
"Bach Edition, Vol. 1" includes the following performers:
"Consort of London, Robert Haydon Clark; Emmy Verhey, violin, Camerata Antonio Lucio, Christine Schornsheim, harpsichord; Neues Bachisches Collegium Musicum, Burkhard Glaetzner Netherlands Bach Ensemble, Krijn Koetsveld, Anton Heiller, harpsichord; I Solisti di Zagreb, Antonio Janigro."
Bach Edition Vol. 17:
Arleen Augér; Annelies Burmeister; Peter Schreier; Theo Adam; Dresdner Kreuzchor; Dresdner Philharmonie; Martin Fläming; Marion Strijk; Systse Buwalda; Knut Schoch; Bas Ramselaar; Holland Boys Choir; Nederlands Bach Collegium; Pieter Jan Leusink The Sixteen Choir & Orchestra; Harry Christophers; Nordic Chamber Choir; Soloists of the Freiburger Barockorchester; Nicol Matt
Mozart Edition Vol. 11 (here's the Brilliant's description on their website):
Mitridate is a brand new production on period instruments, conducted by Early Music specialist Jed Wentz, and sung by rising star Johannette Zomer, Young-He Kim, Marcel Reijans and Alexei Grigorev.
The other two are high-quality licenses from Berlin Classics, with a.o. Peter Schreier, Dagmar Schellenberger, Sylvia Geszty, and conductors Helmut Koch and Max Pommer.
Beethoven (40 cd box):
Staatskapelle Dresden, Herbert Blomstedt - Shoko Sugitani, piano - Berliner Symphoniker - John Lill, piano - Emmy Verhey, violin - Anatoli Krustev, cello - Medici Quartet - Sharon Quartet
In other words, some of these performers are known quantities and likely to be a good value. Others are unknown and--well, unless you want a guaranteed Great Performance, it might be worth the risk and you might discover something wonderful.
I bought the Brilliant Classics box of all the Shostakovich symphonies conducted by Barshai (certainly a well-known performer). All of these performances are more than competitive with higher-priced discs, some are my first choice, and the sound is excellent.