I'm not in any way affiliated with GOG.com or its staff but it is just my duty to warn you about the practices at greenmangaming.com.

They currently advertise a lot of bargain offers and have a 30% voucher for no-one-knows which titles.

"A 30% voucher is available across a great range of
PC Download titles* - snap it up quickly as it
expires on Thursday 3rd January 2013 at 12GMT.

* PC Download games only, some titles may
be excluded. Excludes titles on Cash Back or
GMG Credit promotions. Cannot be used in
conjunction with another voucher.
Voucher expires 03/01/13 at 12:00 GMT"

Guess, where you can't apply the voucher! Yes, usually the titles you want to buy. Later on it tells you, it won't apply to special offers. I don't know what they are, but probably everything.

Then they had Deus Ex GOTY Edition on discount for less than 2€ without an end date and just took it out without notification (Hello GOG aka 'ONLY UNTIL JAN 3' you do the same, I know). You couldn't buy it per PayPal. It was impossible for some mystical reason. Before that they wanted all of my data (address, phone number and so forth).

Summary:
* No proper discount end dates (like GOG).
* Unreliable payment.
* Data-hungry.
* Shady discount conditions.

After that you would still have to deal with Capsule, their own software, but I won't find out how bad that is.

Stay away from GMG!

I bought AIM 2 when it was on sale at 75% off then I used a voucher they had for any game at 20% (which was added after the 75%) and I didn't have any issues with either payment nor their client, I even had to activate it with its DRM which took 5 seconds. However, the game itself crashed every 10 minutes after a while so I didn't bother to continue.

It's a shitty store. Most people use it only to buy steam games cheap.

I had some bad experience with them, like trading in my games even if I didn't click anywhere (it happened to many people, not just me).

I bought Spec Ops The Line for 75% discount plus 30% voucher today. I had no problems. :/

I was about to say the same. Worked alright.

I bought Alpha Protocol for 75% discount plus 30% voucher today. No Steam key was provided. :/

I personally never had the problems mentioned by the OP, but I did hear lots of similar experiences from other people who bought at GMG. Plus some comments about their less-than-impressive Capsule.

Yeah, only use them for some deals like keeveek mentioned. But with today's experience, I might just consider to stop doing that as well.

If they only knew that Capsule is needed basically only to download games, not to play them...

It says out of stock so maybe you'll be provided one when they have more? It says it requires steam so I don't see how you would be able to play it otherwise.

I don't even care so much if the software screws up sometimes, but I hate nothing more like pointless time sinks. Just state offers clearly and tell conditions in advance and everything should be fine.

When they have games marked at 75% off it normally means it's only available at that price for 1 day. Just like Spec Ops: the Line and M&B Collection. (Visibly marked on the top left of the frame). I haven't found anything confusing with their site and have recommended it to others for the great prices. I've never had any problems with paying with PayPal. I've bought several games over the course of months and never had any issues whatsoever. Did you have an issue with your Paypal account in general? That's the only cause I can think of with my experiences. As far as being "Data Hungry" by wanting your contact/address, EVERY SINGLE online store I've come across requires you address and/or info. Amazon, Steam, GoG (though much less specific iirc?), As far as i know this is for legal purposes should something happen. To be angry at this in current times is really just silly imo. Nothing about the discount conditions are shady. Normally things that aren't eligible for discount are visibly marked. Maybe you tried to use the voucher on a special marked promotion and didn't notice? I haven't tried out capsule yet, haven't bought any games that require capsule. So can't comment on this.

If you're having issues with GMG deals (on if they work with vouchers/when they end) then I suggest checking out http://www.reddit.com/r/GameDeals/ . They normally state the the price, if it works with the current discount code, and when it ends.

Edit: Just saw your voucher image. That's really odd, no clue why that is. O.o

Edit 2: Maybe because it can be traded back in. "Excludes titles on Cash Back or
GMG Credit promotions."

It wasn't a 24H deal, no it's not visible in the top left.
What an annoying answer. I did not have in general an issue with PayPal. I thought, you would assume that. I rule out such things.
It's apparently not necessary for every small store, especially when using PayPal.
What is a special marked promotion? Wouldn't that definitely apply to those 75% discounts?

I use GMG mainly to grab Steam games when they're on sale. Works great for me. I pay with Paypal and haven't had any issues with that. As well, the other day I picked up Binary Domain on sale for 75% off and used the 30% voucher on it, bringing the total cost down to $4.37, so I'm not sure what the issue is with that.

*edit* Spec Op:The line is their 24 hour deal at 75% off. I just checked with the voucher and it applies to this game as well. I agree it's not really clear which games it doesn't apply to, but it's easy enough to drop a game in your cart and try applying the code for a further discount.

Well, that's odd then. Never run into that before.

Sorry, I wasn't intending that to be annoying. I just assume very very little when dealing with issues online.

Honestly curious here, what small stores don't?

"Excludes titles on Cash Back or
GMG Credit promotions." Probably the trade in value is considered a credit promotion :/

Anyone who uses Paypal has general issues with Paypal, it's one of the most obtuse, bullshit payment services out there, hanging up payments frequently for loads of folks for days or weeks at a time.

Most people I know use GMG as a place to get Steam games cheaper than Steam. They have some of the lowest prices in the business. Much like Walmart. So you're bound to run into customer satisfaction issues, you get what you pay for.

That being said, I just picked up XCom: Enemy Unknown for $17.50. Not bad. Not bad at all.