TL;DR: unless there are some special circumstances that you didn't specify in your question, that 5.5% is not anything like a guaranteed return.
Don't confuse the coupon rate with the return.
If the coupon rate is 5.5%, that means you'll get a $55 check every year for each $1000 bond you own. (Assuming they pay annually, for simplicity.)
However, it is unlikely that $1000 is the price that bond will fetch on the open market. If the US Government issued a bond with a 5.5% coupon, the market price would increase above $1000. Let's say you ended up having to pay $2000 for the bond. You'll still only get paid the same $55 interest, which means the yield on the bond you bought will be 2.75%. (Note that someone else might buy the same series of bond on the open market and pay a different price -- they'll have a different effective yield.)
If you hold the bond until maturity, and the issuer doesn't default, nothing really changes. But if prevailing interest rates change, then the market value of the bond changes. Say you need to sell the bond in two years -- prior to maturity. Assume rates have gone up, and you'll only be able to sell it for $1500. Your return on the investment in the bond is negative (the $110 in interest doesn't cover the drop in price of the bond). On the other hand, maybe rates drop and the price of the bond goes up to $2200. In this case, your return is your yield amplified by the rise in price of the bond: $110 plus the $200 gain, a 15% return over two years.
If the issuer defaults then your return may also be negative.
Complicating matters even further, if the bond is callable then the issuer may be able to prepay the principal prior to maturity. This could leave you with cash at a time when you can't invest the cash at favorable rates, which reduces your return.