Mixed bag. Generally speaking, a highly technical and critical series, and often quite liberal; and they have published translations of several notable European commentaries. Not really a series that a Reformed person would read and say, "Ah, yes; this truly points out the meaning of scripture." That being said, there are some useful works if used properly (though I can only speak for the NT volumes). Jewett on Romans is often quite suggestive grammatically and structurally (and it's a pretty massive commentary); Bovon and Luz's volumes are highly useful at times. Honestly, I never found Betz's work on Galatians as helpful as its popularity would seem to suggest, but precisely because of its popularity and influence, it's useful to be acquainted with it. Attridge and Achtemeier are both frequently insightful. There are translations of both Strecker and Bultmann on the Johannine epistles - but if you want to read one of the more prominent German's on these letters, I think your time would be better served by Schnackenburg than either of these. Conzelman and Dibelius are certainly "historically" important, though probably less relevant for a confessional Reformed reader.
In other words, mixed bag, but always useful for a good example of current critical trends in scholarship, even more so since German volumes are made available to those for whom they would otherwise be inaccessible. I will say I find the formatting slightly annoying: in several of the volumes, the "footnotes" actually seem to take up more space than the actual text and to be the more important part of the book. The constant moving from top to bottom of the page gets tiring.
Edit
So while I was writing this, far more learned and experienced people answered. Stick with what they said.
The Hermeneia series is, like most other series, a hit or miss series. They do not tend to be conservative. Some of them are so hideously liberal as to be unusable (for instance, Collins on Daniel ripped the text into unusable tatters. I read it. Surely I'll get out of many years of purgatory for doing so). Others are more than usable, they're great, in spite of liberal leanings. Here I'm thinking of Achtemeier on 1 Peter and Attridge on Hebrews. There is no commentary set that would qualify as the only one you'd ever need. As to this one, many of the volumes are worth having, but you have to be able to read critically in order to filter out the liberal biases that are usually present.
