For me what made the DE M900's great were the cheap, £160'ish polymer bodied ones, from around a year to two years ago. However it seems they don't come into stock in the UK much anymore (of course Taiwan Gun have bloody loads of them though) so all you can find are the metal bodies ones so the price get's bumped up by far too much. Are they still good value, I think so (just), but a fair comparison would be another branded gun with a GATE Aster in it (you get the features without the Titan price).
Yes the build quality is not high end, but you get so much for a good price... optical controller with proper cycle control + both piston head and spring guide have bearings + the shimming and lubrication is well done + a very good air seal. But, and you have to be careful here, is that they're not "beginner" friendly, in that the Falcon controller is not tolerant of being used hard on full auto especially on 11.1v so new players may well fry / kill it, plus its battery management is poor and if you forget to take the battery out after a days skirmishing you may find next day a Li-Po with a VERY low cell voltage and possibly even to the point it's been damaged. I love all of mine, but I've been looking after them well (however the M908A's metal receiver is looking well worn now, the paint is very thin / not hard wearing, but I like the look), and I think they'll last me for years yet (I've already swapped in loads of better parts, they're very much "Trigger's Broom").
Personally I'm not sure I'd buy one of their sub machine guns though, mags + some spare parts are a little hard to come by.