This is great for UA elites, great for Marriott (if replies in this thread are any indication). Bad for MR elites, especially MR Golds. As MR Plat, UA Premier Silver is a joke. A more equivalent relationships would have been giving UA 1Ks given MR Silver; or giving MR Gold and Plats UA Premier Gold.
While the 25% bonus miles will be nice, the other benefits are a joke with the possible exception of free bag for those that check bags. Upgrades will never happen (I buy F or J on UA when I fly them anyway) and I don't know about E+ availability but the benefit is at check-in so my guess is middle seat at best on today's full flights.
So for 25% extra UA miles, we are once again degrading Marriott lounge experience by adding more elites to already crowded domestic lounges. I must say that UA management was very adept in negotiating this relationship and Marriott only looked at the quantity of customers at the expense of their elite experience. JMHO.
BTW, for those of you unfamiliar with MR: MR and RCR are identical except for the promotions each programs' members receive. The redemption rates for MR and RCR members are identical at Marriott and RC properties. The lounge access policies are identical, which means no elite access to RC clubs if anyone is salivating. Rules and benefits are identical.