Are recent affordable EEG devices any good?
The Emotiv system has been evaluated in a research setting. Badcock et al. (2013) recorded EEG activity with the Emotiv EPOC and a more conventional laboratory system simultaneously, and found that both systems produced similar results for ERPs with a relatively high signal-to-noise ratio, but it was harder to detect less reliable signals with the EPOC than with the standard lab setup they used.

Reference:

Badcock, N. A., Mousikou, P., Mahajan, Y., de Lissa, P., Thie, J., &amp; McArthur, G. (2013). Validation of the Emotiv EPOC® EEG gaming system for measuring research quality auditory ERPs. PeerJ, 1, e38. fulltext