Just got a 13" and it is my sole computer. I knew that getting it and I had the MBP 2010 13" model but I sold it to get the MBA. I would personally not use it as a primary computer unless it was the Ultimate edition because then I would just get a MBP 13" for less price. The 2.13ghz, 4gb ram and 256ssd are all sufficient for me but Im not sure if it would be if i got the 1.8ghz, 2gb ram and 128ssd.
Im absolutely satisfied with it. Battery life is so conservative when it needs to be, I can play SCII with good quality when i like even though i use xbox live to game. And everything else like multitasking and what not, it is lightning fast. My MBP 13" with an expensive performance SSD was a fraction faster but it did have 2.4ghz and not 2.13. Overall, best computer ive had so far. I actually prefer portability cause i use my computer on the desk, couch, bed, etc. and 13" is plenty esp with the hi-res screen I have no problem reading PDFs and word docs.