Well in my opinion, stelliums only increase focus on a particular house. They are not big like Grand Trines or T-squares but still have significance. Here's a broad overview.
Career-wise, stelliums seem to have a particular influence on a certain area. People with 3rd house stelliums often "need" to communicate on the job; advertising, teaching, writing, singing, or just talking (news anchor, radio host, etc.) are all great possibilities. 12th house stelliums show someone who must work behind the scenes. So on and so forth.
A first house stellium shows a person who is not plain. They have a colorful personality with many sides to it. The more planets, the more lively and different they are. Often they need to display their personalities in the job. Show people who they really are.
A second house stellium, and money money money. Depending on the planets, this person seems to be able to relate to possessions in a personal way. They may not be money-hungry or anything like that, but rather they find they cannot simply "not care" about finances. They value alot of things and have odd tastes. As far as career-wise, a stellium in the 2nd only shows money is more focused through the career, so it's not so big of a deal.
A third house stellium needs to talk. Talk and write. They rarely keep their words to themselves and have a strong sense of what to say. Unless chiron is here or aspects the cusp ruler, expect someone who feels even regenerated by words. Talking just makes them feel better, and often they are great conversationalists with alot of interests and knowledge.
Fourth house stelliums give particular emphasis on home. Most often, a career that either deals with the home (decorater, remodeler, handyman, etc.) or working from the home is indicated, particularly if the moon is here. For these people, everything can come back to family. They are the type who shine at family gatherings and always have something to say about a relative. They enjoy being at home.
When we see a fifth house stellium, huge potential in theatrics or art come into play, particularly if the ruler of the MC is in there. Working with children is often key, and if the moon is here, the teacher is key. These people indulge quite often and have many different pleasures depending on the planets involved. They are great parents and may have many children, unless there are stressful aspects.
Sixth house stellium. The workaholic. These people spend more time on the job than anywhere else. They have a huge interest in health and often go into the healthcare profession, especially if the moon or sun is there. Working gives them good feelings, and they often don't know what to do when given time off. They are excellent employees.
Well, a stellium in the seventh and we see relationships. Often the career deals with associating with the public or public projection. These people have many wants and needs in partnerships and thus, may have many mates before finding the right one. They may work with the partner. They feel incomplete alone.
Dark stuff with an eigth house stellium. Sex may be an obsession if the planets and/or cusp ruler receives challenging aspects. They are good at working with joint finances and make good accountants. Often, the planets involved show signs of people they gain largely from. They need to investigate in the job, regardless of what they actually do. They are consultants, they fix what others mess up.
Spiritual journies are key with ninth house stelliums. These people often question their beliefs constantly and consider philosophy a big part of life. They make great publishers and writers. Often there is a need to have a career on an international level. Travel to many places. If the moon is here, expect someone who shows their talents in as many places as possible.
This is a "take-charge" stellium. These people love being the leader. Tenth house stelliums in women's charts often show a more strict, father-like mother. They are extremely ambitious and have a knack for business. Their minds are practical and they achieve alot if they can harness the energy properly.
"I get by with a little help from my friends". That is so true for those with an eleventh house stellium. They often have friends from all different backgrounds. They generally don't see people in a bad light; they want to be everyone's friend. Often the career is given in some way through friends; getting where they are because of who they know. These people value a good friendship above all, and are probably active in alot of social circles and charities.
Now, last but not least, twelfth house stelliums. These people are talented in the unknown and have great natural psychic ability. They don't do well in the limelight and even prefer working alone. They are interested in the occult. Since the 12th is institutions and hospitals, we see someone who may work in one or both in the lifetime, it's very likely. They often have alot of work in this life, because planets in the 12th show unfinished business.
Hope this helps!
edit: getting extremely long yes I know but indeed Isabelle has a good point once again. Aspects are big. And I myself have seen 4 7th house stelliums, and each person who had one had been in several relationships. So, now I know of 5 people who have a stellium in the 7th who have had many relationships. I'd like to do a study on that, compare charts. See how much validity that has................7th house stelliums............you can see I answered another question about stelliums and mentioned this phenomenon as well.