dgex is most secure but very slow.
bter is fast, iv never had issues with bter and i always use it.
crptsy is crap and having serious issues and i wouldnt advise using it.
poloniex i havnt really used much, seems ok but has been hacked before(they paid back the last people that lost money)

it's easy, just install NXT client and run it.
more info about NXT and FAQ you can find in there - https://bitcointalk.org/index.php?topic=587007.0

it's easy.
just start NXT client.
open Asset Exchange, and look for NEM asset https://nxtblocks.info/#section/assets_exchange/12465186738101000735
Asset Id:   12465186738101000735
click add new asset

you want to buy a nem stake? you can buy on the nxt asset exchange.

don't understand you, what you want, you want to buy some NEM Stakes or what?

2 weeks after NEM will be started you can exchange tokens to NEM Coins, but if you need token now for AE you mast pm PATMAST3R

accept? what you mean?

Update:

Here is stakeholder list population statistics after the fund raising https://docs.google.com/spreadsheet/ccc?key=0AlidWok7hW0DdG1XYWlDemNENl9IaGR5ZHNUNTNhcmc&usp=drive_web#gid=0

Group 1 : veteran account opened before the fundraising.

Group 2 : account opened during the fundraising but showed further activity and engage with the community.

Group 3: accounts opened for NEM registration only at the times this statistics is complied (1 post).

Thread 1 :

Group 1 : 711 stakeholders
Group 2 : 310 stakeholders
Group 3 : 440 stakeholders

Thread 2:

Group 1 : 379 stakeholders
Group 2 : 473 stakeholders
Group 3 : 803 stakeholders   (some already show more activity after this statistics is compiled)

I think this does show the size and strength of NEM community. While we have a lot of veteran accounts participating, we also attract a big amount of new people coming to crypto and joining us.