Yes Yes Yes !
Purchased!

Sold! Come midnight local time (or just a bit after) >_>

Sold!! =D

So, to all you pinball experts out there: which one(s) should I buy and why?

This might be a good chance to try at least one pinball game.

Timeshock is the best one I think.

Yeah, go for timeshock, you wont regret

Ok....and what about that Pure Pinball? It has more tables, but the reviews seem not so good as the pro pinball series.

That's what I call listening to your customer base!
"What kind of promotion do you guys want?"
"Pinball promotion!"
"Well, here you go."
GOG, you rock!

+1
What he said.

I bought Pure Pinball when it was released here, and only played it for about 20 minutes. I didn't like it.
Haven't tried any of the others, but people say they are awesome, so I'll start by getting one of them tonight (probably Timeshock), and if I like it, buy the two others sometime this weekend.

Awesome stuff. I'm buying all the pinball games now.
I had fun with Pure Pinball too, but although you get 3 tables in that game, the features of the 3 combined are pretty much less than the one table you get in the Pro Pinball series.
The Pro Pinball series has been voted the best pinball simulation games of all time, so there. Definitely worth buying every single one of them.

cristianolopes: The "Pro Pinball" tables (which are made by Cunning Developments) are the pick of the bunch. "Pure Pinball" certainly isn't horrid or anything, but it's simply not in the same class.
So... of the Pro Pinball tables, which to get? Everyone has their own opinion, but FWIW...
Timeshock is my personal favourite (and indeed the very best PC pinball table I've ever played). The GOG version runs via DOSBox, however, so you may need a grunty machine to run this one in all its glory. See the forum for details on running this one at higher detail levels than are available by default in GOG's installation: http://www.gog.com/en/forum/pro_pinball_series/dosbox_hires_settings
GOG's versions of Big Race USA and Fantastic Journey are both Windows-native, so performance isn't a concern there (except for the motion blur effect which causes horrific slow-down on my machine for some reason, so you may wish to disable this option).
Fantastic Journey nabs the number two spot in my books, with its friendly nature -- I'm no pinball wizard, and so I really appreciate all the ball-saving features available in this table, as you are far less likely to suffer an unfair drain. The table appears a little simpler, but it's still all kinds of fun to play.
Big Race USA is my least favourite of the three, due mainly to the ease with which the ball drains. I also find the video mode a little dull in this one. The quality is still amazing, and it's easily worth the money if you enjoy pinball, but I play the other tables much more often.
See here for more comments:
http://www.gog.com/en/forum/pro_pinball_series/your_favourite_table
Be aware that you may need to take some extra steps to play the Windows-native Pro Pinball games if you have a multi-core CPU. Windows compatibility mode will basically do the trick (use '95 or '98), although some people found they got some audio 'popping' unless they also set their audio hardware acceleration to minimum.
If you're technically-minded, you may be interested in the alternative steps I took to get those two running to my own satisfaction on my Core 2 Duo / WinXP machine: http://www.gog.com/en/forum/pro_pinball_series/big_race_usa_performance_suggestions
I'll note that Pure Pinball plays well on modern hardware without any tweaking required, but it's worth a little hassle to play the Pro Pinball tables, IMO.
Now, GOG... how about that Windows-native option for Timeshock!?

Make that +2.