I see them at CDNN all the time but never see them mentioned here. Are they any good?

never heard of them... had to do a google:

http://www.thebangswitch.com/microtech-xm-30-e4-magazines/
I have also seen a few bad posts about them on here....

I can get Pmags for $12.99 here on base and they have a bin the size of a garbage dumpter so I have no need to look elsewhere.
Primary Arms has Pmags for sale for $9.99 right now too.

I got 5 of them with my MSAR bullpup.

They work fine for me, in both the MSAR E4, and my AR's.

That said, I don't see any reason to get anything but P-mags.  They're the best.

I bought a bunch of the 40 rounder MSAR mags when they were $10 at CDNN awhile ago. When 40round Pmags are $10 ea I'll buy some of those.

The ones that came with my rifle didn't last 90 rounds.

The newer ones seem to have a better reputation, but again, for the price, I'll take any of the good steel or P mags.

I have been using them in 3 gun matches for about a year now, never had any problems. Local company, and the guys do a lot to support the shooting sports.

The 1st gen msar mag was junk. The current (gen 2) mag seems to be performing very well. There is a James Yeager video out there where he says they have been using them for their classes and have been putting hundreds of rounds thru them with no issues. I have a few, and have been using a clear one as a range mag. So far I've put about 300 rounds thru it with no issues, but I haven't been trying too hard to break it.

All in all, since you can get them for $7 or $8 depending on the sale from CDNN, I think they are well worth the money.

Old mags or new style?

New ones. The guys sponsor our local 3 gun match and we got to try out some out last fall / winter, never tried any of the old style ones. At first i kinda blew them off as i have had good luck with magpul, but after using them as range mags for a while i have been very impressed with them.