I wouldn't call $40 a month for unlimited data expensive or restricted, but that's just me
4400 yen per month is expensive - especially as this is the only real option. I use enough data that it is worthwhile for me, but most of my colleagues with their iphones are using it just for email - tiny amounts of data. They would be fine on a $15-30 data plan from most countries. From what I see - you either use data and have to pay 4400 yen, or don't use any data (or risk getting killed with their crazy per packet pricing).
And the speeds - I have travelled a fair bit with the last three flavors of iPhone (3G, 3Gs and 4), and Japan and USA have been the worst performing networks. I use voip quite a bit in Canada and Europe - it works very well. But in Japan I have given up - the speeds are very slow and unreliable.
Anyway - been here now for the last 9 months - love Japan, but miss the awesomeness that used to be Japanese mobility. I used to buy all the latest phones from Japan (late 90's, early 00's) and was always blown away by what Docomo, Sharp, NEC were doing. I have my 3GS on Docomo (no packethodai) and my iPhone 4 on Softbank (consistently slow, seem to limit voip and other services)... I have an Experia on Docomo that is pretty lame. I will probably swap out my iPhone 4 for a Galaxy s - as the Galapagos phone is a disappointment .
Certainly not global leaders any longer. Hope they catch up - the world could use some of their NFC and iSeg expertise