Warning, I'm an AA cell-a-holic. There are other options open to you that can pack even more output into a small package but I refuse to consider them because I am sold on the AA format specifically in Sanyo Eneloop (and comparable) low self discharge rechargables. Cheap, reliable, easy to work with. All my opinions on lights are shaped by this bias.
For road biking I use a Planet Bike Blaze 2W bike light *and* a Fenix LD10 R4 on my drop bars. I ride frequently at night both for commuting and pleasure, almost year round. It's rainy here seemingly most of the year (I jest) but in particular in the late fall/early winter the combo of dark and rain means good lighting is critical in this big city. We are also fortunate to be able to ride to country side in about 10 minutes from our house so streets with no lighting, or well gravelled trails, are also part of any night rides. A typical ride at night for me is 20 - 40km, sometimes that much each way if going to a destination.
The Blaze is a 2xAA cell (side by side format) light, $34 CAD. We have a few of them around here for my wife and kids bikes. They include an easy to install quick release mechanism, have a low/high/strobe series of outputs and are plenty bright on their own for most city and limited country (no street light) riding. At 34$ it is one of the better low cost bike-specific choices out there in my opinion. I always reach for one of these even if I also have my Fenix light on board, just for convenience and redundancy if nothing else.
It puts out light in a more controlled beam width than most LED flashlights do.
Here's a comparison done by someone else between the Blaze 2W and an older Fenix L2D (2xAA slightly older tech now) flashlight and the Blaze 2W. That shot doesn't show what the author probably saw with his own eyes - more spill on the Fenix.
I've been meaning to do a similar comparison between the Blaze and my every day carry light the Fenix LD10 R4, will try to do that in the next few days if the rain ever stops here.
I'm not really sold on 2xAA "in-line" on the handle bars of my roadbike because I find the form factor too long. A 1xAA light typically sits extremely solidly in a TwoFish lockblock (a nifty attachment system, cheap, but you can make your own with a length of old inner tube). Not only that 1xAA doesn't protrude back into the cockpit area of your bike. I fear that much overhang there could get in the way when climbing (standing) vigorously. This may not be an issue for you, and indeed a great many people ride with 2xAA lights and they give you the great advantage of more run time without stopping to change cells where I have to stop after an hour and a half (if running on high output) and swap a cell.
My own evaluation of whether a 1xAA really works for me *for riding* is still on-going. For regular commuting I find it ideal. For long training rides - it is nothing to swap a cell out once during the ride. In the long run I'm trying to determine what sort of alternative lighting I can use for doing very long rides which include substantial night riding (brevets - 200km, 300km, 600km and more). Swapping cells out several times a night may not be so appealing then.
But for other purposes I generally much favour single cell lights, so I'm motivated to try to make that work to get the most bang for my bucks.
The only other light format that interests me is a 4xAA format for a general purpose usage. There's a possibility this could find use on a bike. Higher output levels over a longer run time are the main reason I'm considering this but I'd be buying such a light (the soon to be shipping Fenix LD40) for other purposes first and foremost so won't be devastated if the light proves to be too long or too heavy for regular use on the bike.
At this point I'm pretty happy with the Fenix LD10 R4 - it is bright and very good runtimes - generally leading the class for comparable lights right now. In the daytime on gloomy days I run it in strobe mode.
Incidentally in the rear of the bike I now run a Portland Design Works Radbot 1000 high powered blinkie. The Planet Bike SuperFlash is another popular rear blinkie - we have 6 of those; when the Radbot came out I had to get one as the local reseller has them on at the same price and they are easily double the output of the SuperFlash. Both are good, I would not hestitate to get a SuperFlash if the Radbot is not available to you. The Radbot will chew through AAA cells somewhat faster than the SuperFlash.
If I were to look at 2xAA lights as an alternative the LD20 is the one I would probably turn to at present, again because I'm always keen to think of these as general purpose lights and being involved in emergency management and disaster response I'm always considering emergency use as a core use which means I need lights that know how to run well on regular alkalines in a pinch. The LD10/20 are industry leading in this regard.
Another light that is soon to be released that has the 1xAA format, runtime and output characteristics I like is the Zebralight SC51 (recent review on CPF) - when it comes out it will challenge or beat even the LD10 R4 for output and runtimes on all common AA cell types including NiMH, Lithium, and Alkaline. This interests me for reasons other than biking of course, but one downside for bikers: no strobe mode.
PS: Often strobe mode on flashlights with a "tactical" heritage are nauseatingly fast. If this is true for whatever light you buy you may find that you can only run it in that mode during the daytime.
PSPS: On the specific Romisen you've mentioned
you might just find it is too narrow a beam. Try to find some beamshots comparing it or other riders with same just to be sure if you do go that way. Ideally you want some spill so that you can see things on the side of the road just about to run/walk/jump into your path. I quite often surprise skunks and raccoons on my rides - if I only had a narrowly focussed spot beam in front of me I'm 100% sure I'd have hit some of these at 20 - 35kph by now. Messy.
and am looking for a light to attach to it, but i realise that i'm not going to get a dedicated bike light, within a reasonable price, plus i could do with a strong flashlight for other purposes.
