My experience - for what it's worth:

These drugs all work for me.  I had one nerve spared (Oct 2001) and was
making good progress in the ED department before undergoing salvage EBRT
last summer.  We were getting to the point where we didn't often need
pharmacological help but following the radiation we are back to relying on
them most of the time.

Viagra - I can split 100 mgm pills into quarters or even sixths and they
work fine but - only on an empty stomach (their "Good Morning, Good Morning"
commercials are appropriate).  No food, no booze, no side effects.  They
take effect in about 20 minutes.  Best "bang for the buck" since they are
cheap when split up into crumbs.

Levitra - I haven't been impressed when splitting the 20 mgm pills but whole
pills work every time and produce a very solid pre-prostatectomy result in
10 - 15 minutes. Afterwards I get blue vision flashes from them and feel
somewhat light headed for a couple of hours after.  I can use them in the
evening on a full stomach and a couple of glasses of wine (but not more!)

Cialis - Works as advertised for a good 2 days.  Not as firm but basically
available on demand.  They don't work for me if split them so I use a full
20 mgm dose.  I like the spontaneity aspect but they sometimes give me nasty
muscle pain in my leg that starts 3 days after I take them and takes at
least a day to get over.  Any over the counter pain killer works to manage
the pain.  I get this side effect only about one time in 5.  For a few days
afer taking one of these I wake up - how shall I put it? Let's call it
"bushy tailed", wink, wink, nudge, nudge - say no more.

This is my personal experience and I would be surprised if it is the same
for anyone else for that matter.

I also found Viagra works best on an empty stomach. If you eat late
like we do, you can take it before dinner.

Cialis has a longer time span, but the erections don't seem as firm. A
little help from a cockring fixes that problem.

Both seem to work better with some supplements. ArginMAX and pycnogenol
specifically. The following article is where I learned about this:

http://www.findarticles.com/p/articles/mi_m0FDN/is_4_8/ai_111304013

Treatment of erectile dysfunction with pycnogenol and L-arginine
Alternative Medicine Review,  Nov, 2003  by R Stanislavov,  V Nikolova

Penile erection requires the relaxation of the cavernous smooth muscle,
which is triggered by nitric oxide (NO). We investigated the
possibility of overcoming erectile dysfunction (ED) by increasing the
amounts of endogenous NO. For this purpose, we orally administered
Pycnogenol, because it is known to increase production of NO by nitric
oxide syntase together with L-arginine as substrate for this enzyme.
The study included 40 men, aged 25-45 years, without confirmed organic
erectile dysfunction. Throughout the 3-month trial period, patients
received 3 ampoules Sargenor a day, a drinkable solution of the
dipeptide arginyl aspartate (equivalent to 1.7 g L-arginine per day).
During the second month, patients were additionally supplemented with
40 mg Pycnogenol two times per day; during the third month, the daily
dosage was increased to three 40-mg Pycnogenol tablets. We obtained a
sexual function questionnaire and a sexual activity diary from each
patient. After 1 month of treatment with L-arginine, a statistically
nonsignificant number of 2 patients (5%) experienced a normal erection.
Treatment with a combination of L-arginine and Pycnogenol for the
following month increased the number of men with restored sexual
ability to 80%. Finally, after the third month of treatment, 92.5% of
the men experienced a normal erection. We conclude that oral
administration of L-arginine in combination with Pycnogenol causes a
significant improvement in sexual function in men with ED without any
side effects.

John