iBonds are amazing. I started buying in my early 30s (I'm mid-30s now). If I have to redeem them when I'm in my early 60s (due to the 30 year limit), well, that's likely a low tax year for me (I anticipate being at least semi-retired by then), and if it isn't, oh well. For all the time between purchase and redemption, they serve as an excellent tier of an emergency fund. I keep maybe three months' expenses in various cash, but my job is reasonably secure anyway.
They're my second tier and part of the overall allocation that, taken as a whole, might appear to outsiders as too conservative for many (I'm at approx age - 4 in bonds, including these). At the moment, I have over seven months' expenses in iBonds and haven't made the purchase for 2018 yet. When I do, I will have almost ten months' expenses in iBonds (although some won't be liquid yet if the emergency happens immediately after purchase I suppose).
Are they possibly a net loss due to being taxed on redemption, including interest-related gains? Maybe. But I'm more and more into the belief that one's portfolio isn't so much stocks and bonds as stocks and conservative investments, and iBonds fit the latter category. If you don't like calling them bonds, call them inflation-indexed cash, which they pretty much are.
They also serve to sort-of kind-of expand my tax-advantaged space. Mostly they have a weak tax advantage: I buy them with post-tax money but the gains are taxed, but not until I redeem them, which again I anticipate being in low-tax years, at least for any bought in my 30s. Maybe my RMDs will change the low-tax nature when I have to redeem the ones I buy in my 40s, but maybe that just means I have a year of high tax on the redemption but use that to pay tax to convert more pre-tax retirement to Roth to reduce future RMDs. Who knows.
Long story short (too late), I think they're great, especially since I max out tax-advantaged space and have more to put somewhere.
"What was true then is true now. Have a plan. Stick to it." -- XXXX, _Layer Cake_