(see the 2014 update)
About one year ago I was tired of the problems with my then-current internet provider, Play. So I decided to try the 4 major operators in Poland. I got a "starter pack" for each one, which can be bought for about 10 zl each, and tried them with my telephones/modems/computers. These are the results of testing with speedtest.net, pingtest.net, youtube's speed test and torrenting some big Linux disc image. And since this is mobile internet, I tried each in a set of places in Warsaw - and some even outside of the city.
Note that I was looking for smartphone use: voice + data, with data being in the 5 GB/month range, possibly more. So data packages measured in MB didn't cut it (since they are usually much more expensive). And internet-only SIMs were out, too.
The tests are one year old, but I can confirm that the results are still valid at least for T-Mobile and Play, since I still use them both.
For non-technically inclined people, the summary is: get T-Mobile. Play is unreliable, but cheapest. Orange is surprisingly crappy. And Plus didn't even have anything serious for smartphones (that is, for voice + data "in big quantities").
And the gory details:
Era / T-Mobile: the best one. One easily reaches the advertised speeds, while testing and downloading. Pingtest gives marks between 4.0 and 4.2 (C, sometimes even B). Pings between 160 and 190 (!), Packet Loss 0, jitter typically in the single digit (!), sometimes up to 20. Youtube averages 19.
It's not even the most expensive of the operators, if I remember correctly. The problem is that once you use up your monthly allowance, you can NOT extend it. So if you need more than 5 GB, you're toast. Their internet offer is advertised as unlimited, but that only means that after you use up your paid package, you can continue using internet at about 2 or 3 KB/sec. Practically unusable if you can't limit yourself to text.
Era is my main operator, so I can confirm that it still works well - even if I had some complaints at some moments because of weird connectivity problems, which were quickly and personally answered - even if in a useless way. :P
Play: the cheapest internet operator. Too good to be true, certainly. Half the price of T-Mobile, can be refilled as you wish, there are even options to use internet during the night without it counting against your allowance. But, even before getting into the numbers, it is plainly shitty. On average, more than once a day the telephone or modem will loose the connection; sometimes in "standard" ways ("the PPP server has stopped responding"), but more frequently the phone just suddenly shows no bars of network reception. Both the telephone/modem and the computer wait for it to come back, which never does - I have tried waiting for hours. You can just restart the phone and it will connect again, but sometimes in less than a minute it will happen again. I started thinking it was some problem with my telephone, with drivers, with the mac... But, it has happened with different computers (although always macs), with different telephones and modems, even using a VM with Windows to drive the modem. Same thing always. So, I can NOT recommend Play at all, for any serious internet usage at least. No idea about how it works for voice. And maybe for short connections, like those typical in a smartphone would be usable. I wouldn't risk it, though.
Anyway, I also tested it "numerically", and the results were: pingtest marks between 1.6 and 3.9 (F to D). Pings between 140-250, jitter between 80-260, packet loss 0. The max standard speed is 125 KBps. There is an option to get 400 KBps, but I feel lucky if I get just half of that.
I STILL use Play for "bulk internet"; it is so cheap (even more so with the free nights option) that it always gets me to try again. But it also always manages to get on my nerves; 3 times already in 4 years it has made me prefer to pay extra to other operators... and the 4th is coming soon. SO annoying, SO unreliable, SUCH helplessly blank faces in the hip shops when I tried to get some info. Fuck them.
Also: Play only provides HDSPA connections. If at any moment the phone has to fall back to EDGE, you're as good as with no coverage. That was the case when I tried it at my workplace, which is only about 2 km away from the city center: I couldn't even connect.
Orange: the only operator which showed packet loss (!) in pingtest. Marks between 1.1 and 3.9. Pings up to 440. Jitter between 18 and 205. I never reached the advertised speeds for downloads, but did for uploads. Keep in mind that I tried in different locations, with different modems and telephones, so I can only think that this was simply Orange being just shitty.
Plus: this was my first operator in Poland, and maybe that's why I had something of a soft spot for them. And it was impressive to be able to work with SSH and VNCs almost all the way in the train between Warsaw and Tricity. With Play, you'd loose coverage before you leave the city!
Alas, when I tested them a year ago they didn't have any specially competitive offer. In fact the only data+voice / smartphone-related offer they had was something tied to BlackBerries (heh, I wonder how are they doing now with that?). It was funny that the shopkeeper seemed rather desperated about the smartphone questions; it felt like I was n-th person to ask the same thing... and to be surprised that they had no options.
Anyway, I also tested with pingtest. Marks between 1 (!) and 4.25 (!). Pings between 134 and 388. Jitter between 13 and 104. No packet loss. Easy to reach advertised speed.
So, when looking for reliability and sanity keeping, T-Mobile seems the only one commendable. Also, with that small jitter, Skype and such were great. People prefer videoconferencing when I'm in the phone via T-Mobile than when I am in the computer via Play...
What I can't understand in T-Mobile is the lack of "refills": I just want to give them more money, after all :P. But, no way. So stupid.
A few more details: when testing with Pingtest.net, I used the auto-selection of test sites. Each operator seemed to consistenly cause a different test site selection, so I guess it was the best one for each of them, surely according to ping times.
It's remarkable that T-Mobile has such a low jitter (and consistently so), that Orange reliably suffered packet loss (and was the only to suffer it at all), that Play is so damn unreliable, and that Plus... well... HELLOOO!! wake up and smell the 21.1 century!