I've discovered that I can find the exact same multi-leg UA trip on US for hundreds less. Can I book the whole trip on US using my UA MP#?
And get credit for UA metal when I'm on a US codeshare?

Yes on everything. The only thing you miss out on is the ability to upgrade in advance.

Thanks! You da' (mahasamat)man!

Also you miss the ability to do OLCI about 33% of the time based on my track record with US codeshares on UA. US tends to mess up the e-ticket so that it gets out of sync with the PNR, and neither OLCI nor kiosk will work. TA at the airport checkin counter can fix it in 30 seconds, or you can waste 30 minutes on the phone with 1K web support in India who still can't fix it.

Otherwise no problems, miles post fine including RDM, EQM, and BIS/Lifetime. Except once in a while the US booking class doesn't get re-mapped to the UA booking class. A cheap US fare class like E might map to a UA class like T, and that's how it posts normally. However, I have had US deep-discount "G" class fares for UA codeshares, which are valid US classes for UA miles, stay as "G" when it posts with the UA flight number. G is not a miles-earning class for UA (it's a travel industry class), and I went through days of calls/emails to 1K before they finally fixed it up as the actual UA class (S, if I recall correctly).

Watch out for those two problems and you should be ok.

This plus OLCI seem to be only downside with US, per my limited experience.

Same is not true of other (*A) deals - some things cannot be done on .bomb (note: this is ~1 year old, may be fixed)
- ANA - cannot select seats
- LH - cannot lock E+
- SG - cannot select seats

In above cases, had to sort it out @ check-in counter. They did recognize 1P/1K status, and was even able to get courtesy upgrades in some cases.

Also, with code-share you get LESS miles than UA metal - the "elite bonus" miles do not post... only route/actual and fare bonus. Hopefully this has changed

The OP's case is UA metal, so the above doesn't apply.

Yesterday, I booked a seven leg MR to PIT, via a US codeshare on United metal, for $145. The .bomb quote was over $450.

Same goes for bmi. I booked a flight with a BD flight number and saved $4200 over the UA price for the same flight (purchased business class). Haven't flown it, yet, so can't speak to the check-in issue which will be different anyway since it's international. However, I can say that I, and a few other posters on the bmi forum, have noticed the online seat map is screwy. Most seats don't even appear on the map, only a select few that are already assigned. And even though the phone agent assigned me my preferred 8J on the 777 return, I got put into 12J. Seat map associated with the flight showed 8J available, but not the seat map associated with my PNR (the seat's gone now, so it's a moot point -- I'm still in C, anyway).

Booking a trip this fall SFO-LHR, and all the BMI codeshares with United are coming up $400 less than United quotes. If I book via BMI, will I be able to use a SWU in advance given it is United metal? If I call up United 1K desk, will they help me out, or will they re-direct me to BMI customer service?

No. You can only upgrade codeshares at the airport, not in advance. Not a big deal as most of my SWU upgrades clear at the airport anyway.

just reviving this old threat, I cannot seem to find clarification of the following questions before I possibly book a BD codeshare flight on UA for the first time, again because it is around 400USD cheaper to SFO!

1. If it is in W class, will it earn 100% normal miles on UA, plus my elite bonus?
2. if I enter my mileage plus number in the booking, will I be able to see it under my itineraries on UA's website, and select my preferred economy plus seating?
3. Anything else I should know that may disadvantage me by booking it via bmi?

Thanks very much

1. Can't speak authoritatively to this one. It should, but my J (Z) ticket hasn't given me my class-of-service bonus which I'm supposed to get -- one of the items on my "to do" list is to puruse this. I did get my elite bonus.

2. You'll get a different PNR for UA, but yes, you can look it up. Might have some problems with the seat assignments; see my earlier post.

3a. Should be no problems, but I've read a couple of horror stories on the BD boards. Just be sure to print out both your confirmation email from BD and your e-ticket receipt from UA, just in case.

3b. I did have some trouble with checkin. When I tried to check in online, the system told me there was no electronic ticket issued (despite both airlines telling me it had been). Couldn't check in at the kiosk because of a "schedule change" (which I had acknowledged 6 weeks earlier); had this issue both directions.

I took advantage of this myself on Friday. Booked LHR-LAX-SAN for �343 vs �643 on .bomb!

1/ 100% miles in W, yes. Not sure on the bonus though.

2/ Probably not. Well not at least straight away. There appears to be a "feature" with the BMI website (that is mentioned on the BD forum), where even though you add your MP number at the time of booking, the BMI website can drop it from the reservation. I had this problem myself and had to phone the BMI helpdesk and ask them to add the number for me. After that, the itinerary almost instantly appeared on .bomb.

3/ As mentioned previuosly, the seat maps will be either messed up or not available. However, you can simply phone UA Reservations and tell them what seats you want. I did this on Saturday and got 21H & 17A^

So in answer to your question. For the savings involved, it's worth booking on BMI.com. Assuming of course that you don't mind dealing with potentially two call centres.......

Not for Z I think

Wow, if you can book on bmi.com, I would be Really impressed. Most people have to use the airlines website. Does bmi.com have a secret booking engine?