Do you have to be a bit up on your linux skills to get the most from this phone?
Not at all, you can be a lazy git like 99% of the owners that will buy the N900 (me too) and just sit back and wait for the thriving community to develop all the cool stuff that we want (the key is it is a very open platform so the community can do things with it and if you felt up to it you could too, the option is there). To use as a smart phone doesn't require any linux skills at all.
For me so far, only a couple of days of use I am finding;
Pros
1. The keyboard is really easy to use, very quick and not at all as troublesome as I thought it might be.
2. It is very nippy, I have not seen the slightest bit of lag.
3. Great touch screen and I find it very easy to use though not as smooth as my iPod touch which I presume is just like the iPhone?
4. The browser is just ace

5. The lack of portrait, except in phone mode doesn't bother me in the slightest.
Cons
1. The battery life isn't epic at all but I was expecting that and I'll simply charge it every night whether it needs it or not and I suspect it sometimes would not need it.
2. The bleeding alarm clock has no volume control and gets a bit too loud for me, I like to be woken up with a softer volume than I am getting from it.
3. Music wasn't easy to get on, but maybe I was being a bit of a drip, in the end I sync'd it with WM12 on Win7, went on a treat but my album art is missing and I don't give a flying about that, I listen to music I don't look at album covers... In the end it was so easy that this could easily have been a Pro.
I see people on the Nokia forums moaning about not having MMS etc, for gods sake if you are going to spend £500 on a smartphone/tablet/mobile do some research, it staggers me to see people moan about that.
I did my research and knew about its pitfalls, no MMS, poorish battery life and I was prepared to put up with them.
I have to say my experience with linux is very much like you skeeter, try it, think it's cool for a bit, try to do something that usually goes very badly wrong and then give up and on goes the Windows lol. I am a .NET developer and I have recently upgraded my lappy to 8GB ram and a 640GB hdd, have installed VMWare Workstation 7 for some SharePoint 2010 dev work and I am seriously considering downloading the Maemo SDK and seeing if I can get into doing a bit on the darkside.....
Don't know much about the HTCs or the Milestone that you are looking at but I wouldn't let your lack of linux skills hold you back from purchasing a N900,
good luck with whatever you choose,
J.