I think the Tchibo stick is the easiest option (unless you already have a UMTS card that's not simlocked), but if you need coverage outside large cities, it might not always get you connected.

I was okay at my parents' house, but not at my friend's (and she lives fairly close to Koeln). Out in the country=forget it with o2!

That's because O2 roams on T-Mobile outside urban areas.

I'm going to stay in the Frankfurt Exhibition area.

I suppose it is one of most "covered" area (but also heavily loaded, I hope to be able to get enough band).

O2 covers 99,9% of Germany's population with GSM/EDGE and some 60% with 3G/HSPA. Further for some rural regions O2 still has a unilateral national roaming agreement with T-Mobile, which will last until the end of 2009.
So if you run out of coverage with an O2 SIM that's really bad luck. All in all T-Mobile and Vodafone still provide better coverage but unless you leave civilization O2 should do the job. Also keep in mind, that the cheapest prepaid offers on T-Mobile and Vodafone are their dayflats for € 4,95 per day - that's nearly € 150 for one month, which costs € 20 with Tchibo.

Does that mean I should have been able to connect through T-mobile in those areas (I thought I remembered something about the roaming)?
I tried many times, but it was a no-go.

I was very surprised to not get a connection at my friend's house, who lives very close to Koeln.
There were many not covered areas along the train route from Duesseldorf toward the coast (Norddeich), but it *is* a very thinly populated area along the way.

Frankfurt should be no problem at all!

The roaming is switched off from area to area starting last year already. By the end of this year it will be completely disabled. While o2 is still building a lot of GSM stations these days there will be for sure areas without coverage with o2 (at least temporarely) towards the end of 2009.

Chris

Ah, no wonder!
Thanks for the explanation.

Coverage inside long-distance trains is an issue, since modern IC (InterCity) and ICE (InterCityExpress) wagons have metal-coated windows and a steel frame, which severly attenuates radio signals (up to 30db in a ICE). Allthough a lot of waggons have so called intrain-repeaters installed, older ones do not work for O2 and eplus, as these two operators use EGSM900 and GSM1800 but older repeaters do only amplify the PGSM900 band and eplus' portion of the GSM1800 band. 3G is not amplified by those repeaters at all. According to press releases german rail operator Deutsche Bahn is about to swap those old GSM repeaters, but if you're in a waggon without repeater or with an old repeater, you'll experience bad network coverage.
Upon request seats can be reserved in waggons with repeaters, which carry this symbol:
http://upload.wikimedia.org/wikipedi...andyaussen.jpg
For more details in German see Intrain-Repeater ? Wikipedia

I have read all the posts and I am quite close to buy Tchibo.

I am a frequent traveller all around Germany and ask herewith if there is somebody else with such a profile of usage.

Experiences? Are you content?

Thanks