A couple of thoughts on the M203:

1.  Hell yeah, they are cool.  it is a little more awkward, but mounting it on an M4 carbine clone is totally worth it (and the adjustable length stock is helpful for shouldering).  Stand alone stocks are handy, and you can always swap back and forth.  Personally I like the "barrel mount" version as opposed to the picaitnny mount version of the M203.   The 9" and 12" barrels get the same range out of them, so all things being equal, the 9" is nice for a little less weight.
2.  DS Arms is out of stock of receivers and doesn't expect to have any for a year or more.  LMT is more expensive, but if you are going for an M203, things are not going to be cheap, just accept it and pay the piper.  
3.  Chalk Rounds, and even components for chalk rounds, have become hard to find and expensive since ATF has gone kind of goofy in the last year.
Even last years chalk rounds could be had for $5-$8 each and now you are looking at closer to $13-$15 each. General consensus is that ATF has determined that M203 chalk rounds now count as "low explosives" and need a powder magazine for storage if assembled.  If the blank is not installed it just counts as a blank and an inert 40mm round.  Yes, stupid as hell, no, it doesn't make any sense, but it has made it hard to get chalk rounds.  Some guys and companies now sell components including rounds that just need the blank launching cartridge installed, and they often sell those with the "inert" chalk rounds.
4.  Be aware some ranges may not be cool with you using and M203, you may need private land, preferably somewhere with at least a good half mile clear and where they aren't going to care if you leave the zinc pusher (the round that gets launched is a blue plastic "egg" and a cylindrical zinc base for weight with chalk inside) in their field where a plow or farm implement might hit it.  You will most likely NOT find the zinc pusher even after looking for it, they bounce off anywhere and everywhere.
5.  With practice you can pop a man sized target (steel, dummies, etc)  with chalk at 100 yards- painting things with orange chalk is fun.  
6.  Chalk rounds can and will maim and kill if you strike a living thing, and the slow travel makes them arc shot, drift in high wind, and they can easily skip off the ground at a shallow angle- that scene in Generation Kill where the guy accidentally skips the round off the ground and it tags a local in the head and kills him is 100% accurate.
7. Safest technique on an M4 is to use the offhand to pull the trigger and use the mag as a forward grip, strong side hand on the pistol grip of the carbine.  This is more difficult balance wise than gripping the barrel with the offhand and pulling trigger with the strong hand *but* if the barrel lets go on the M203 and you are holding the barrel- well, it is better to just hold it without your hand on the barrel.

Not trying to scare you off, but just want you to know some things to be aware of- As long as you understand what you are getting into and what the requirements are for a safe place to shoot them are, then yes, they are awesome.

Sven
Manticore Arms