I think rewards cards are great! For the card to be worthwhile, you need to pay the balance in full each month, and you also need to make sure you don't buy anything that you would not have purchased if you had to pay cash. If these two things aren't a problem for you, not using a rewards credit card is essentially throwing money away on each and every purchase you make.
In general, my personal preference is to get cards that pay cash back (or points that can be redeemed for cash) instead of other things like airline miles, hotel points, etc. One reason is that you can redeem for cash at lower levels (often as low as $20) instead of having to save up for a whole airline ticket. Another reason is that the base rewards level on a lot of these cards is either 1% cash back or 1 airline mile per dollar spent. With 1% back I know I'm getting a penny back for every dollar I spend. With miles, it's harder to tell. Sometimes you can get lucky and find an award ticket where the cost in miles is low enough compared to the retail value that you get more than a penny of value out of each mile, but often this is not the case. I'll usually take the guaranteed penny now over the mile that may or may not end up being worth more than a penny at some point in the distant future.
One thing I would recommend is to get a few of these cards to optimize your rewards. I regularly rotate between three cards. I have a Barclaycard Rewards MasterCard that gives 2% back on gas, groceries, and utilities. I have an Amazon.com Visa from Chase that gives 3% back on purchases from Amazon.com and 2% back on restaurants, gas, and drug stores. I also have the Chase Freedom card that gives 5% back on a different category each quarter (currently gas; Amazon.com and certain department stores next quarter; other times it's restaurants or groceries or something else). All three cards pay 1% back on all purchases outside of their special categories. By using whichever card gives me the best cash back for each purchase, I am able to get an average of pretty close to 2% back on all my spending. I may have to look into adding that Quicksilver card to bring my default rewards level up to 1.5%.