Ailey troupe impresses - and disappoints.

They are a people's company. They present dance in a broad sweep of styles and forms - superbly, and with sparkle.

But does their dancing have artistic depth - that power of feeling, that ability to provoke and change one's perception of things?

This is the question that perennially faces the Alvin Ailey American Dance Theater, one of America's most popular performing troupes.

After seeing two performances during their recent appearance here at the Wang Center for the Performing Arts, I would answer usually.

Artistic pluses or minuses aside for the moment, this is a remarkable institution. The company celebrated its 25th anniversary this year by going out on a grand tour around the US, a three-month marathon of performances that saw them in places as far away as Anchorage, Alaska and Miami Beach, Florida (Boston was the second to last stop).

It has been a fitting tribute to a major force in contemporary American dance. In its quarter-century of existence, Ailey's company has:

* Appeared in 44 countries to almost universal acclaim.

* Been largely responsible for the development of today's black dance, with its emphasis on ethnic and socially oriented choreography - the basis for such Ailey masterpieces as ''Revelations,'' ''Blues Suite,'' and ''Cry.''

* Built one of the premier dance schools in America, one which has opened the doors to artistic achievement for many less privileged youth.

Mari Kajiwara, one of the company's leading dancers, is Ailey's personal assistant - the individual responsible for setting and maintaining his works as he conceived them. Sitting at her dressing-room mirror, the woman whom Ailey calls ''The Keeper of the Steps'' explains the company's repertorial approach. ''One part of his (Ailey's) great genius is the recognition that one (choreographer's) vision, no matter how fully realized, is not the broad spectrum.'' In practice, that means the troupe dances not only Ailey's works but those by many different choreographers working in a wide variety of styles and forms.

But does this breadth produce depth? Ms. Kajiwara defends the company: ''Our aim is not to be trendy, even though we've been accused of that.''

Unfortunately, some of the pieces presented - standards from their repertoire - do not bear that comment out. ''The Stack-Up,'' by Talley Beatty, is the worst offender. Backed by funk-music hits like Earth, Wind, and Fire's ''Faces,'' this work presents urban tableaux - scenes from today's inner city. Much of the company is dressed as if it just came out of a disco. The style of the dance - disco-jazz - is entertaining and well-executed, but the plotline is really too thin to be effective or satisfying. The result is a piece that might have been more popular when disco was at its peak; now that that craze has ended, this dance has lost some of its appeal.

Several other works presented weren't trendy, but neither were they deep or powerful. ''Suite Otis,'' by ''The Wiz'' choreographer George Faison, even saw the company fall somewhat from their usual high standard of smooth, well-timed ensemble work.

On the other hand, Ailey's new ''Isba,'' set to George Winston's enchanting piano solos, is gorgeous. The company - costumed in shades of pink, orange, and purple - turns and leaps about the stage in what becomes a moving tapestry of colors and dancers.

''Treading,'' by Elisa Monte, is another standout. Backed by the surging, almost hypnotic minimalist music of Steve Reich, it features the superior talents of Ms. Kajiwara (who has danced with the company for 14 years) and Keith McDaniel. ''Treading'' is a piece open to many interpretations, as the two move in what seems like a slow motion film through off-balance leg and arm extensions and then into a intertwined, rolling duet. At times they seem like protozoa in the deep, dark ocean, at others like two pieces of moving modern sculpture.

Certainly, between the two of them, they show an unmatched mastery over the body in this work of demanding off-center movement. And Monte's choreography is a significant addition to post-modernist dance, as effective as it is unusual.

Two other works that showed the most artistic depth were Ulysses Dove's ''Night Shade'' and the same choreographer's restaging of Ailey's ''Precipice.'' The first delves into African tribal themes, namely the ancient practice of freeing oneself from evil spirits through ritual. The pace of the dance stays at a frenzy throughout, the dancers running, jumping, and throwing each other around a circle of covered candles. The finish is especially powerful: after the evil spirit has finally been destroyed, the dancers cleanse themselves by dunking their heads in a huge tub of water. Thanks to ingenious lighting, the water - flinging upward from their heads as they pull them out - becomes beads of streaking silver light.

The story line in Ailey's ''Precipice'' - of a star pop musician who is self-destructing (program notes refer to Jimi Hendrix and Jim Morrison) - gives the company the artistic focus that it seems to lack in other pieces. Gary DeLoatch is ideal as the cocky, self-assured star, prancing about the stage to the equally ideal jazz music of Pat Metheny and Lyle Mays. The company - one half his fans and the other half his friends - swirls around him, tugging and praising and demanding his attention. But it is DeLoatch who gives this piece its throttling power with his utterly convincing portrait of the self-consuming forces of passion.

The result is that, in ''Precipice,'' Ailey's touch for humanity is elevated to highest effect. As Mari Kajiwara says, ''It is his use of people as people that's made him universally popular.'' If only he - and his company - could use this talent in a deeper, focused manner more often.