Disclaimer:It could be that this only applies to the Ruhrgebiet/Düsseldorf area in Germany, and maybe only to myself. I have also zero science-grade equipment, or knowledge about mobile networks.
I’ve got a 3G USB stick for my private notebook, which I primarily use during work (I can’t/am not allowed to/don’t want to connect the private box to the corporate network) and on the train. I just changed the data plan from simyo to klarmobil, and the latter is clearly the better one.
Both data plans cost 10€/month for a unlimited data amount, where the the definition of “unlimited” differs quite a lot. The simyo plan lets you transfer 1GB per month, and when it’s spent, you buy the next one for 10€, not matter if the GB or the month finishes first. Klarmobil lets you transfer 500MB at full speed and then throttles to GPRS speed. I normally use up 200-300MB, so that’s not an issue. Both plans can be terminated on a monthly basis and do not cause any monthly extra cost.
Simyo is a brand of e-plus, klarmobil is o2. And that’s the real difference: e-plus does not offer HSDPA, and the network coverage is much weaker. Between Düsseldorf and Duisburg, the network fades on and off. The o2 network is a LOT faster, and significantly more reliable. So for me it’s thumbs up for klarmobil.