I lost my SGSII and needed a budget phone to take me to the end of my contract. Went with the G300 due to the screen size, cpu, battery, and importantly (!) the modding community at MoDaCo.
If you think this phone is too cheap to be good, you'd be wrong. Its down to the absence of a brand name (and possibly, as I read somewhere, a subsidy by the Chinese govt).
Looking at the G300 as about one third the price of a new SGSII (£340 on amazon), its more than one third as good. To me that says good value for money.
That said, its been a bit of a comedown for me. They're not comparable in all but the most basic terms - screen and battery life are similar, they both have Play Store access and decent browsers. But cpu and ram are both lower - noticeable when browsing a couple of tabs or desktop websites, switching apps. And the touchscreen is not as responsive (I'm scoring less on Angry Birds!).
I'm not putting the G300 down, in fact I'm praising it due to what they packed in for £100. Obviously its not as good as a phone 3x the price, but if you are looking at £160 or below it is THE BEST. Better than the Monte Carlo, the Ace, Wildfire S, etc. Use http://www.gsmarena.com/compare.php3
to compare specs. I was looking for 4" screen min, 1Ghz cpu or more, and min 512 RAM. The storage built in is a bonus, as is the battery capacity, 5MP camera and flash.
As an aside on the locking discussion: I'm on T-Mobile - considered getting a SF2 and putting my T-Mob sim is, but the sales guy said it would be locked to Orange.
Assuming he is right (big assumption given my experience with phone salespeople!), then the SD will be locked to Orange.