Alder Stratocasters have a full bodied sound. They usually have a bump in the midrange with an otherwise flat response from the lows through the highs. The warmer of the two common Stratocaster body woods. Excellent for the three pickup / single in the bridge position models as they tend to rein in the brightness. A great choice for the player in a Dino three or four piece / one guitarist band as they provide an excellent full range guitar tonality and enough high end to pull front and center over keyboards, bass, or drums. Alder Strats excel in smoothness.

From Guitar Player Magazine Online

Alder. As with ash, it’s impossible to discuss alder without making reference to Fender, which first used alder prominently in the late ’50s and early ’60s. It’s a medium-weight wood, although quality cuts of alder used for guitar bodies will often weigh less than denser cuts of ash. Alder has a strong, clear, full-bodied sound, with beefy mids and excellent lows. Its highs sizzle slightly, but are rarely harsh, and it offers a decent amount of sustain. Slightly brownish in its natural, dried state, alder’s grain isn’t necessarily unattractive, but it usually isn’t particularly interesting, either. It is typicaly used under opaque finishes, but some examples can look good under darker translucent finishes. Like ash, alder is most often used on its own as a body wood.

From John Suhr 

Alder – Rich and full, Alder is strong in the lower midrange. This is probably the most widely used wood in the 60's for 3 single-coil style guitars. It is medium to light in weight and takes well to sunbursts and some transparent colors.

From Warmoth

Alder (Alnus rubra):
Alder is used extensively for bodies because of its lighter weight (about four pounds for a Strat® body) and its full sound. Its closed grain makes this wood easy to finish. Alder's natural color is a light tan with little or no distinct grain lines. It looks good with a sunburst or a solid color finish. Because of its fine characteristics and lower price, Alder is our most popular wood and it grows all around us here in Washington State. The tone is reputed to be most balanced with equal doses of lows, mids and highs. Alder has been the mainstay for Fender bodies for many years and its characteristic tone has been a part of some of the most enduring pieces of modern day contemporary music.

From USA Custom Guitars     

ALDER - The "tastes like chicken" of wood. It has a nice even response all the way around the tone spectrum! Mostly known for its great Blues and Rock tones. It's the one body wood you can go play a Death Metal Gig one night, country the next night, Blues the next night and it's sounds fine for all of them. It is normally lightweight and has a closed grain which is a finishers dream. We carry hand selected Alder that is suitable for clear finishes as well as bursts and solid colors.

From the Frank Falbo DRG Tonewoods article

Alder

Alder is light in weight with soft tight pores like Basswood. But there is a large swirling grain pattern to it with harder rings and sections. So imagine a Basswood type texture but with harder rings peppered throughout. That adds to the stiffness, and the complexity of the tones. It retains more of the highs that Basswood softens, but also gives some room to the lows. You have a broader spectrum of tones, which leads to the perception of a little less mids than Basswood.

Note the Fender Artist series and customshop* series of guitars. The following models use Alder for the bodywood.

Malmsteen Strat
Jeff Beck Strat
SRV Strat
Eric Johnson Strat
Clapton Strat
David Gilmour*
Robin Trower*