I don't know whether this count as a serious test or not, but I was up half the night re-encoding a UK Sci-Fi radio series (almost 15 hours worth) which I'd originally encoded with 3.97 Final in VBR at -V6 to fit onto one CD-R for listening to in the car.
With 3.97 Final, it came out at 717MB and made very rare usage of the 320Kbps rate according to EncSpot.
With 3.98 Beta7, -V6 was clearly going to overshoot my desired total encoding size by around 10% so it would appear that we now have an increased average bitrate in the lower -V settings although mine and wootabega's tests earlier showed a slight under-shoot at the -V2 and -V3 levels.
I started the encoding process again, but this time at -V7. The total encoding size for the whole exercise now came out at 711MB vs the 717MB at -V6 with LAME 3.97.
Once again, more frequent use was made of the 320Kbps rate. There is a distinct peak at 112Kbps with the distribution of the remaining blocks being biased more heavily toward the high end of the graph than with 3.97 Final.
According to EncSpot, the upper cut-off frequency was 15kHz with 3.97 Final at -V6 and was 14.5kHz with 3.98 Beta7 at -V7.
Although I haven't carried out any proper ABX comparisons, voices definitely sound slightly less granular to me personally with -V7 with 3.98 Beta7 than at -V6 with 3.97 Final.
For what it's worth, this new Beta gets a big thumb's-up from me personally for low bitrate VBR encoding.

Cheers, Slipstreem.

EDIT: I forgot to mention that the new Beta makes much more use of SS during stereo encoding than 3.97 Final which tended to be much more heavily biased toward MS than SS according to this test. This may just be a trait of 3.98 in general as I'd never taken much notice before.