I'm the proud owner of a LM31 and it's been my EDC for around half a year. I'm quite satisfied with it -- it's definitely a no-nonsense EDC that I wouldn't shed a tear over if I lost it (as opposed to some of the pricier alternatives).
The beam is good, the runtime is good, and the price is right. Also, I might be the only one on this side of the planet who appreciates this, but the UI is so simple. Click low, click high, click off. None of this "half push the butt end and twist the top 1/4 turn and back" stuff that sounds like you're playing that Bop-it stick thing that was popular a few years back.
I don't, however think it's Fenix quality though, mine came with a defective tailswitch and I don't think I'm the only case of that on these forums. However, both Lumapower and BatteryJunction were excellent help and got me a replacement free of charge. After that, I've had absolutely no problems with the light and I use it heavily (and have dropped it multiple times).
You might also want to look at the OLight T15 as an alternative to the LM31 if you want a step up in brightness, runtime, and features. IMO the T15 really nice (I just ordered one), but it's not good bang-for-the-buck when compared ot the LM31.