I have bought thousands of dollars of CDs from ArkivMusic over the course of the past several years. It is the most convenient source I know for searching out and buying classical CDs.
As for the ArkivCDs that are burned on demand, I have bought at least 30 of these over the course of the past year. While I have not been able to compare the quality of the sound with the original issues, I have been VERY impressed with the sound quality of many of these. As an example, the live 1965 performance of Mascagni's opera, Cavalleria Rusticana, conducted by von Karajan at La Scala in Milan (Deutsche Grammophon # 419257) is stunningly good.
Unfortunately, this particular CD set is of the type that you mentioned with no liner notes or booklet. However, please note, many of the ArkivCDs do come with a high quality booklet and/or liner notes that are a reasonable facsimile of what was issued with the original release. Just one example is another ArkivCD opera recording that I recently purchased: Delius, A Village Romeo and Juliet, Charles Mackerras, Austrian Radio Symphony Orchestra, 1989, Decca # 430275. On the web page that provides the detailed information for any given ArkivCD, there is a statement right before the individual track listings indicating whether liner notes are included.
Schipo, in response to your question, why buy these ArkivCDs?, for me, it's just a matter of the unavailability of the original pressings. Arkiv Music is only licensing the right to do these pressings for recordings that are no longer in the catalog of the record company that owns the recording. I am personally thankful they are offering this service, as many of these recordings are nearly impossible to find even if you are willing to undertake the laborious task of scanning the on-line sites for a used copy.
I don't expect that the ArkivCDs offer better sound than the original pressing; however, based on the sound quality I have heard on many of these discs, I do expect that the quality of the ArkivCDs is probably every bit as good as the original CDs issued by the major labels.