I recommend Blocktrail.com as default explorer. Blockchain.info is not really good and their mobile UI doesn't show transaction details correctly. Blockr.io is also good and in fact, better than Blockchain.info mobile UI.
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We also noticed during the test that of the 5 block explorers MultiBit HD provides, 3 had problems viewing our transactions. This was observed even though they were definitely out in the network and had been received in the destination wallet. These were: biteasy.com, blockchain.info and bitpay.com. The other 2 block explorers had no problems viewing our transactions. These were: blockr.io and blocktrail.com - so here's a shout out to those providers.