which Fast Fifty
The one that will extend your repertoire most is probably the Macro lens, which can also double up for landscapes and as a longish portrait lens.
Best regards, John
DA*55 = $650ish US
Nearly double as good?
The change would make a nice deposit for that other prime...

I would not part with the FA 50!
http://www.pbase.com/iberg
FA 50/1.4
Compact (220g and 37mm long) time-tested performer. Widest FOV of those you specified. N/A from SRS but cheaper than other choices.
DA*55/1.4
Has SDM focus mechanism, Quick-Shift Focus and Weather Resistant (WR) build. Larger (375g) and "non-standard" 58mm filter (all the rest here are 49mm). £549 at SRS.
DA 70/2.4 Limited
Extremely compact (130g and 26mm long) but don't forget to add the hood. Quick-Shift Focus. Significantly slower than above and narrower FOV. Excellent Limited build. £465 at SRS.
D-FA 100/2.8 Macro WR
Still fairly light but huge in comparison to the other lenses here (340g, 81mm plus long hood). Rounded diaphragm blade. WR of course and macro you don't get from the others, but narrowest FOV. I wouldn't say this should be your first prime and comparing it with "fast fifties" is problematic. £495 at SRS.
If you are interested in such a wide range of glass I wonder why you omitted the two best choices? These are the reasons to shoot Pentax. They are also full-frame compatible with proper aperture rings, etc. for film use. If you find them expensive compare to fine glass from Leica, Nikon etc. and realise they are relative bargains. Buy them used.
FA 43/1.9 Limited
Tiny (155g 27mm) piece of magic that is the sharpest lens available and the best fast fifty ever made. £699 at SRS.
FA 77/1.8 Limited
Tiny (270g 48mm) telephoto -- my favourite lens. £779 at SRS.
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Only half a stop difference but that means you can shoot at 2.0 and it's sharp ith great bokeh. For the money no question. I love both but unless you are loaded go for the FA 50 and put the rest towards another great lens.
http://frogfish.smugmug.com/ Pentax. Pentax DA*300/4, Cosina 55/1.2, Lens Baby Composer Pro & Edge 80, AFA x1.7, Metz 50 af1.
Nikon. D800. D600. Sigma 500/4.5, Nikon 300/2.8 VRII, Sigma 120-300/2.8, Zeiss Distagon ZF2 21/2.8, Zeiss Distagon ZF2 35/2.0, Sigma 50/1.4, Nikkor 85/1.8, Nikon TC20EIII, Nikon TC14EII, Kenko x1.4, Sigma 2.0

Matt
Shooting the Welsh Wilderness with K-m, KX, MX, ME Super and assorted lenses.
I strongly suspect that Pentax have upgraded the SDM technology a lot since the earlier much-reported failures, as all the reports I read seem to relate to older designs (and are often lenses bought some years ago).
Like you said.
The SDM was seems to only be a problem with the first two lenses that had it, the DA*16-50 and the DA*50-135. Never heard of much trouble with the DA17-70, DA*55, DA*200, DA*300 or DA*60-250.
I believe one of the professional Pentax user (forget his name) posted a comment that he send his Pentax DA*16-50 and DA*50-135 back to Pentax to get the SDM upgraded so it might be true they are using different SDM these days.
btw SDM, HSM or any kind of these motors do wear so most likely you will need to send it back to the manufacture to have it replaced with new ones. Don't know how fast they wear though, they are probably good for a couple of decades so don't worry about it too much.
Stefan

K10D, K5
DA* 16-50, DA* 50-135, D-FA 100 Macro, DA 40 Ltd, DA 18-55
AF-540FGZ
It depends very much on what you want to shoot.
The one that will extend your repertoire most is probably the Macro lens, which can also double up for landscapes and as a longish portrait lens.
Essentially I am seeking a "faster" lens for use under normal lighting conditions. In this respect all three meet the requirement. I have to decide which offers extra opportunities above my kit lenses and in this respect I think , as John says, the macro would be the option. However I am leaning towards the "fastest" the 50mm 1.4 which can be had for £254 from microglobe and leave me funds towards the macro at Chrissie before the VAT rise.
The LBA is quite frustrating illness and one I fear will never be conquered

Howard
Body: K7 of course !!
Lenses: DA18-55WR,DA50-200WR,FA50-1.4
Ian
Secondhand SMC F 50/1.4???????????

Say again?
From what I've read and was told by SRS when deciding between the two lenses, I'm sure the FA50/1.4 is also an excellent lens. However, I bought the DA*55 in summer 09 just before the price hikes, when it was much more reasonably priced and only around £50 more than the FA50 so it was a fairly easy decision for me at the time. Whether you want to spend £549 on the DA*55 when considering your other options is a tough one, but I would certainly recommend it as a lens.
Alternatively, if you have a budget of up to around £550 and can pick up the FA50 for £254 as you mention above, you could always consider buying the (non-WR) DFA 100mm macro as well - I believe it can be found for just under £300 at the moment.

Cheers, Adam
Body: K5
Flash: AF-360
Primes: Pentax DA 35 f 2.4, Pentax DA*55, DFA 100 Macro
Zooms: Sigma 10-20 f 4-5.6, DA 17-70, DA 18-135, DA 55-300, Sigma 50-150mm f 2.8 (Mk 2)
hkwiles
Member
Body: K7 of course !!
Lenses: DA18-55WR,DA50-200WR,FA50-1.4