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Note:

You can exchange 30,000 bonus miles for $300 for United Airlines air-ticket.

Other possible rewards include:

$250 in "Lettuce Entertain You" Dining Gift Cards
OR
$1500 in dining gift certificates (15 X four $25 gift certificates for 30000 miles) from Restaurant.com.

Terms:

Pay no annual fee for the first year you are a Cardmember, a $95 value.

25,000 Bonus Miles:
With this bonus offer you will qualify and receive 25,000 bonus miles after your first use of the card. First use includes using your account for purchases, balance transfers, or any checks that are used to access your account; cash advances are not included. After qualifying, please allow 6 to 8 weeks for bonus miles to post to your account. This one-time bonus offer is only valid for first-time United MileagePlus Explorer cardmembers with new accounts. Previous and existing United Airlines cardmembers/accounts are not eligible for this bonus offer. United MileagePlus Explorer cardmembers will only receive one 25,000 bonus miles award if they apply for both the United Explorer Card and the Continental OnePass Plus Card after 7/19/2011. These miles do not count toward elite status. If your account is not open for at least six months, United and Chase reserve the right to deduct the bonus miles from your MileagePlus Program account.

5,000 Bonus Miles:
With this bonus offer you will qualify and receive 5,000 bonus miles after your first use of the card and addition of an authorized user to your account; both must occur during the first 2 months from account opening. First use includes using your account for purchases, balance transfers, or any checks that are used to access your account; cash advances are not included. After qualifying, please allow 6 to 8 weeks for bonus miles to post to your account. This one-time bonus offer is only valid for first-time United MileagePlus Explorer cardmembers with new accounts. Previous and existing United Airlines cardmembers/accounts are not eligible for this bonus offer. United MileagePlus Explorer cardmembers will only receive one 5,000 bonus miles award if they apply for both the United Explorer Card and the Continental OnePass Plus Card after 7/19/2011. These miles do not count toward elite status. If your account is not open for at least six months, United and Chase reserve the right to deduct the bonus miles from your MileagePlus Program account. Additional cards and all cardmember communications will be mailed to the primary cardmember, who is responsible for all charges made by the authorized users. Reference to the account may appear on the authorized users’ credit reports.