Yes, the Hoover Dam is a must-see. Make sure you park inside ($7 per car) and walk to the outside. There you can walk the full length of the dam, view its immense depth from the top, gasp in horror at the receding waterline of Lake Mead, marvel at the beautiful art deco architecture and design. If you choose to just drive through the bridge (doesn't cost anything), you won't be able to see anything as they built barriers along the side to prevent any gawking unless you have a neck like a giraffe. Trust me, I've done both, so fork over the $7. The oohs and aahs will be worth it.
We did the full dam tour ($30 per person) but we didn't care for it much. Skip it unless stuff like turbines and power plants excite you. Ehhhh. We just like pretty views, so just go to the visitor center and on to the observation deck.