I code css and html on an iPad using Textastic which is a ftp program which is also a good code editor with colour for tags. I have ordered a 128GB Pro and am also wondering about a touch or type cover. I have no problem with the on screen keyboard on the iPad and will wait before purchasing a cover.
I don't believe the issue between on-screen and physical keyboards is one of having "problems" (unless the language layout just doesn't work out), but rather one of pure efficiency. You're very unlikely to reach, say, 80 wpm with an on-screen keyboard, whereas a physical keyboard is simply more efficient--far more screen space, better typing angle, more natural feel, and most likely fewer errors since people are taught on traditional keyboards.
So really, the proper question in relation to "how much typing do you want to do" is "how efficient do you want to be?" Sure, I
can type a 40,000 word novella using an on-screen keyboard with few actual problems, but I certainly would never want to.
For minimal web coding where you're in an existing framework/infrastructure, it probably works out fine. But developing a C++ application from scratch with millions of lines of code--probably not. Then somewhere in between: YMMV in terms of "acceptable efficiency."