First, the good news. Prices in Spain and Portugal are generally reasonable compared with other Western countries, though your spending power may depend on your exchange rate at any given time. And on the whole, the introduction of the euro has been a blessing for visitors to Spain and Portugal. Prices in pesetas and escudos just had too many noughts to be manageable, whereas figures in euros are at least on the same order as pounds and dollars.
Less welcome is what to expect if you need to deal with banks. First, don't expect service with a smile, it simply isn't part of the Spanish or Portuguese work ethic. Then, most Iberian banks are only seriously interested in getting their grubby paws on people's savings and disdain casual transactions such as changing money or cashing traveller's cheques (and we are talking about high-street banks, here, not merchant banks). In fact, many now offer the latter possibility only to customers with an account, as though they were providing a service. Some even refuse to allow you to open an account unless you have your salary paid into it. And the standard of service varies enormously from branch to branch of any given bank.
In general, savings banks (called cajas) are a better bet (though not much). Like their equivalents in other countries or building societies in Britain, these organizations were originally semi-charitable institutions intended to help working-class savers buy property, for example. In recent years, however, they have heavily modernized to compete with the banks and some have overtaken them in terms of wealth and clout.
For safe access to money, I suggest that ATMs are your best option, being ridiculously thick on the ground and relatively cheap, though this may depend on how your own bank treats foreign currency withdrawals (many US and some UK banks charge a "conversion fee." I definitely recommend that you avoid traveller's cheques, which are really unnecessary for Spain and Portugal, are costly and can be nigh-on impossible to cash.