Definitely two tiers of lights here...
Tier 1: 3W and less. Usually all plastic housing (the old Magicshine being one notable exception), and self-contained batteries. Most have traditionally produced a tightly focused spot beam, which is bright when view from nearly straight back, but move off-angle and the intensity falls off very rapidly. Pretty much any of these lights are good for night time use, but even the best of them should be considered only marginally adequate for the challenging, brightly lit daytime conditions. The Cygolight Hotshot Pro80 attempts to address the weakness of a single spot lens by replacing it with an elliptical lens. This spreads the beam over a wider and flat angle, which is ideal for taillights, but when you spread the same amount of lumens over a larger area, the perceived intensity will go down. There are a few other makers that have picked up on the need for a wider beam in back, and it may have been the NightRider Solas that introduced it first in this "tier" of lighting. There is even a Chinese light (called the ESSEN EL 601R) that uses the dual lens technique with one spot and one elliptical, however, the power just isn't there. Kudos for the right idea though! One thing to keep in mind about the elliptical lens is that it MUST be oriented with the ridges perpendicular to the ground.
Recently there are some "in-between" lights that toy with the $100 mark (such as the Specialized FLUX taillight). The Specialized is unique among this group in that it uses indirectly focused light for a more efficient use of the available lumens. A very smart design, but not particularly friendly when it comes to mounting flexibility. Although not new to the taillight world, the NightFlux RedZone is also as step above most of the lights in the same category as the Pro80. The RedZone takes the shotgun approach with NO focusing optics, which throws light in every direction, but is not particularly efficient for a bike-mounted taillight. GREAT for the helmet though!
Tier 2: 6W and up (DesignShine, Dinotte, and a few others, possibly ORFOS), but you have to pay for the extra power. These usually require a metal housing with heat sinking features (heat is the great killer of RED led efficiency) or some other method of extracting heat. Some of these lights cannot be used safely at night (such as the Dinotte Daytime Red). Most (not all) use an external battery to get effective run times at the higher power. The higher output can produce higher intensity over a MUCH wider beam compared to Tier 1 lights. So not only are they much brighter, but they offer a greater "field of view" so you can make your presence known even sooner on that winding 2-lane road. They also provide a much longer range daytime awareness. Some will grab your attention at a MILE or more in bright sun! Cost is definitely a factor, but it's just like buying insurance... only you can decide how much is enough.
With the amount of distractions that can vie for a drivers attention and/or prevent a cyclist from being seen in time to be avoided, I prefer to generate enough light power out the back to IGNITE the cars behind me, and then back it off just slightly..
