Considering a Dual SIM phone for 1 number + 1 data SIM. But does it matter?

[inquisitor]

Originally Posted by international

In your experience, could you use a dual SIM phone for 1 number + 1 data extensively (because of pricing policies in different countries) or dual SIM was not a benefit for data, only for 2 numbers?

I recently switched back to a single SIM phone as Wifi and GPS sensitivity of my Zopo ZP980 (dual SIM) were inacceptable. Nevertheless I used the Zopo ZP980 for half a year and while travelling I used it exactly in the scenario you are asking for - while the primary SIM slot would host my German home SIM card, the second was for local SIM cards which gave me affordable data and cheaper local calls. This worked flawlessly as you can select at will which SIM to use for data, voice and sms (the latter can also be selected case-by-case when calling or sendings texts respectively).
Though I cannot recommend Zopo because of their obvious quality issues their phones are special in their ability to assign the 3G-capable radio module to any of the SIM slots from the menu. All the other phones I'm aware of have their 2G and 3G radio module assigned firmly to one of the SIM slots which will probably result in more frequent SIM swapping.
For more details see the thread Andy has already linked above.