Most of my itineraries are based on aircraft type where I have a choice.

My first time on a 777, I extended my connecting time in DEN to get one of the first ones back to LAX. This was 1996. I rode in United First (my first time in a 3 cabin plane). They had a 2-2-2 configuration but the seats were wide and oh so comfortable and there was plenty of distance between you and the seat in front of you so that your seatmate did not have to get up to let you out. I miss those seats, the new United First Suites are far less comfortable than those; however, I still choose United's 3-cabin 777 over other options.

The biggest disappointment is when there is an unplanned equipment change. In 1998, I had a day trip DEN to IAD back to DEN and then on to LAX. DEN to IAD was a 744, my first, no problem. The return was to have been the same 744, but I got to IAD and found out there was an equipment change. They had to dust off an old DC-10. On the 744 I had window seats in United First. These too were spaced enough where you didn't have to bother your seat mate to get out; however, it was not the same on the DC-10. I paid full F fare for this flight, when I found out about the equipment change, I asked to be put on the non-stop to LAX (a 744), which they refused. On the DC-10, they put me in a window seat, and I asked for an aisle which they did not have. This plane had definitely been out of service for awhile. The movie didn't work properly (no PTV's), old cloth seats, and limited overhead storage. The 744 was to have been 3 cabin, but the DC-10 was 2 cabin; therefore, those of us foolish enough to pay F fare got the same service as Business class customers got.

I prefer the 777 over the 747. I will take the A319 over the 737; however, I do prefer the 757 over the A320. I hate the 767's and will avoid them whenever possible. The RJ's are OK for flights under 90 minutes and can get a seat up front. I would love the opportunity to try a Dash 8 again. Rocky Mountain Airways used to fly those into my home town and only got to fly it once.

Yes, i did. But it was a long trip - more than 20 hours. And i also had lot of discussions with the airline concerning the seat number. They wanted to give me a seat on 767 upper-deck but "unfortunately upper-deck is completely full".
So i flew 747-400 Upper-Deck, then 767 (but was same flight-number as the 747). But nobody was able to tell me what plane-type is flying the 2nd part of the trip. "We'll see".
I don't like flight numbers where you have to change plane!!!

Yep, try to do it whenever I book a flight. Last December whilst in Sydney I booked a 3 day trip up to Cairns. I deliberately booked the Qantas flight that had a B743 scheduled there & back, rather than flying with Virgin Blue who happened to be about $20AUD cheaper each.

The travel agent thought I was a bit weird for doing so.

Ned.

And when i'm flying economy-class i would chose a 330, 340 or 767 - wouldn't fly the 747 - very bad seat-configuration.

I went DFW-HOU-DFW on a CRJ-700 cause I wanted to ride the CRJ and I like that Aircraft. It was a nice ride too. That's what I do when I ride somewhere, I usually go by aircraft and mabye if the price is good I can try that one or go with something else.

Lots of times. Even chose two connections over a non-stop so I could get a new plane and carrier. Flew LAS-LAX-SFO-SEA so I could catch PA 747 on the SFO-SEA leg. When I worked for AA and was non-revving on business to LHR, I'd frequently take the 727 to JFK so I could take the 747SP to LHR.

It's one of my priorities every time I fly somewhere. 2 years ago I chose Air France to fly to LAX, just so I could fly on the 777. Last year, coming back to Australia, the best part was flying on a CX A330-300 from SYD to HKG - I did not book the flight personally since it was a student exchange trip, but it made me all the happier!

Of course, nothing will ever top choosing BA001 just to fly Concorde - in fact, flying to New York and straight back home just to experience Mach 2  

I have a long list of types I WANT to fly on before it's too late, from the Tupolevs to the DC-10 and the MD-11. I might just try to convince my parents that we should fly NW next time we go to the U.S., so I can get a DC-10 on my logbook  

all the time...back when DL was flying the tristar i would only leave PBI when there was a tristar flight, which of course meant that i was always connecting through atlanta. and i booked i flight to LAX on DL specifically so i could get on the ATL-LAX MD-11 flight. and in Feb. i paid more to fly air france to CDG over DL so i could get on the B777.

it's a sickness i think.

I used to pick my flights based on aircraft type a lot more so back then I than I do now. Nowadays, it's more a matter of convenience and sometimes what available fare buckets are left, since I sometimes book at the last minute. But admittedly, prioritizing aircraft types regardless of flight schedule (and consequently adjusting my itinerary based on chosen flight schedule) was a hell of a lot more fun. I just wish I had the time nowadays.  

Most definitely yes!!!

While most of my flights are chosen by this criteria, I am desperately trying to organize a routing (European) that would get me onto an F100.

Some flights I have taken on the basis of aircraft type include:

DEL-MAA-KUL-SIN to try out the MH A330-300 in 2000
SIN-BKK-DEL to try out the TG B777-300 (HS-TKA / HS-TKB) 2000.
FRA-CPH-DEL for the SK MD-90 & B767-300 (2000)
BOM-DEL - AI 747-400 (2000)
DEL-LKO - S2 B738 (VT-SIK) 2000
DEL-LON - BA B744 (2002)
CDG-LYS - AF A319 (2002)
LHR-FRA-DEL - LH A300-600 / B744 (2002) for my son.
DEL-FRA - AI B743 (Shivaji)

One of my most memorable series of flights purely to try out the aircraft type was in a single day in 1997, I flew FRA-FDH-FRA-BRU-HAM-FRA just to fly on the ARJ, A310 and A321 of LH. Having done that, I managed to fly in every aircraft type that LH then had in its fleet (except the historic JU52 DAQUI). I am now also trying to get onto a flight with the LH/A330 to keep up my personal score.

My next trip planned purely on the basis of aircraft type is DEL-DAC-DEL (Biman B733 and A310) coming Monday/Thursday.

Yes, I always try to avoid any A320 family flights since they have tiny seat windows that give me limit of view!

Aside from safety, fleet-type-utilized is my #1 criterion for choosing an airline... will always seek out airlines which operate twinjets on their longhaul routes as opposed to 3,4engined types.

Last year I had to go down to Washington and I specifically took the USAirways Shuttle and the more expensive 9 AM Shuttle only because it was being flown with an A-320 a plane I hadn't flown with yet. Also when I had to go between Denver and Phoenix I connected in Los Angeles so I could take a 737-800 and ERJ-140, both of which I hadn't flown before, this also necicitated me getting up at 4:20 AM but I did it!
DC-10's Forever

This is probably common but when scheduling my flights to ABI I will generally take an extra hour layover to fly on an ERJ rather than a SF3 turboprop. AA Eagle usually has 4 SF3 flights from DFW-ABI and 3 ERJ flights daily.