Gemara

R. Yehudah's wife bought wool and made a nice garment. She would wear it when she went to the market; R. Yehudah would wear it when he prayed.

When he wore it, he blessed 'Blessed is He who clothed me with a mantle!'

Berachos 59b (Beraisa): If one built a new house or bought new Kelim, he blesses Shehecheyanu. (If they are for him and for others, he blesses ha'Tov veha'Metiv - Gra deletes this.)

He blesses ha'Tov veha'Metiv only if he has a partner (who shares the benefit).

Support (Beraisa): The rule is, if it is (only) his, he blesses Shehecheyanu. If it belongs to him and someone else, he blesses ha'Tov veha'Metiv.

Rishonim

Rosh (Berachos 9:16): The Ri says that we bless only for important Kelim, similar to a house, but not for minor things such as shoes, Anpil'os (small shoes) and a linen shirt. I say that it depends on the person. An Oni might feel more Simchah over a shirt than a rich person feels over important Kelim. We find that R. Yehudah had only a dilapidated cloak, and he used to bless on it '...who clothed me with a mantle', for it was as important to him as a mantle is to a rich man. The Yerushalmi says that one blesses even on a worn-out garment if it is new to him.

Hagahos Maimoniyos (Berachos 10:1): We must say that the Yerushalmi discusses when the old Kelim are somewhat important, like new Kelim.

Beis Yosef (OC 223 DH v'Isa): This is when they are as important to him as new Kelim.

Rebuttal (Bedek ha'Bayis): We need not say that they are important. 'Old that are new to him' could mean that he never owned these before!

Rambam (Hilchos Berachos 10:1): One who buys new Kelim blesses Shehecheyanu, whether or not he has similar Kelim.

Poskim

Shulchan Aruch (OC 223:3): One who buys new Kelim blesses Shehecheyanu, even if he already has similar Kelim, or if he bought more than once.

Beis Yosef (DH Kosvu ha'Tosfos b'Perek): Tosfos (Sukah 46a DH ha'Oseh) says that Rav Shrirah Gaon says not to rely on the Gemara in Berachos, for we say Shehecheyanu only on something that comes from time to time. We follow the Rif, Rambam, Rosh and all Poskim, who say that we bless on new Kelim. The Rashba (1:245) says that one should bless Shehecheyanu whenever a son is born, even if he already has sons, for the Halachah follows R. Yochanan according to Version #2. I saw many Gedolim who do not. It seems that they hold that anything that does not come (regularly) from time to time is Reshus (optional) to bless. If so, the same applies to new Kelim.

Magen Avraham (5): I did not find people blessing. Perhaps they rely on what Tosfos says in the name of Rav Shrirah Gaon. One should not be lenient (rather, he should bless). This is only for Kelim that one uses. One does not bless for Seforim, for Mitzvos Lav Leihanos Nitnu (fulfilling Mitzvos is not considered Hana'ah). The only Mitzvos we bless on are those that come from time to time. However, Hilchos Berachos of Rabbeinu Meir says to bless for Seforim.

Kaf ha'Chayim (20): The custom not to bless is limited to gold and silver ornaments and utensils. The custom is to bless on garments. On other things it is best to exempt them with something else, or to bless without mentioning Hash-m's name and Kingship.

Mishnah Berurah (13): Kelim includes garments and utensils for eating, drinking, etc., if he gets Simchah over them, based on his wealth. The Chayei Adam says that if one desired a Sefer and was happy to buy it, he blesses. The Berachah is for Simchah, not for usage. Therefore, we do not protest against one who blesses.

Kaf ha'Chayim (25-28): It is proper for both parties to bless Shehecheyanu upon completing Shiduchin, and at the time of Kidushin (and Nisu'in). Also their parents should bless on the Nisu'in, for they fulfill "take women for your sons..." It is proper to bless Shehecheyanu upon writing a Sefer of Chidushei Torah l'Shem Shamayim, and all the more so upon publishing it, or for paying to publish another's Chidushei Torah. He brings Torah to the world. This is no less than buying Seforim! One who was appointed leader or Rav, or merited to move to Eretz Yisrael, should bless. One who reaches 70 years should bless. In all of these cases, the Machazik Berachah says to bless on a new garment or fruit. If he cannot, he blesses without Shem and Malchus. The same applies to every Safek or Machlokes.

Shulchan Aruch (ibid.): When he wears it, he blesses Malbish Arumim.

Mishnah Berurah (18): He blesses Malbish Arumim before Shehecheyanu. If he wears it in the morning, he can be Yotzei with Malbish Arumim in Birchos ha'Shachar.

Kaf ha'Chayim (32,33): Yafeh l'Lev says that nowadays we do not bless Malbish Arumim. We are Yotzei with Malbish Arumim in Birchos ha'Shachar. Also, Malbish Arumim connotes that the person has no other clothes. It is not appropriate for one who has clothes and acquired an outer garment. The Ben Ish Chai says that it is best to wear it the first time in the morning, to be Yotzei through Malbish Arumim in Birchos ha'Shachar. If he must wear it in the evening, he blesses Malbish Arumim without Shem and Malchus. Likewise, it is best to wear a new hat or belt in the morning, to be Yotzei with Oter Yisrael b'Sif'arah and Ozer Yisrael bi'Gvurah in Birchos ha'Shachar. If not, he should say them without Shem and Malchus.

Shulchan Aruch (6): We do not bless for something unimportant, such as a shirt, shoes, or Anpil'os. If he is an Oni and feels Simchah over them, he blesses.

Magen Avraham (9): This applies only to wool or similar garments that people do not buy frequently.

Rema: Some say that even an Oni does not bless over a shirt, shoes, or similar things. This is the custom.

Beis Yosef (DH Kosvu ha'Tosfos Sham): The Terumas ha'Deshen (36) says that one should follow Tosfos (ibid.) and not bless on totally unimportant Kelim. People often get new shirts, shoes, and Anpil'os. One blesses on Kelim with some importance, e.g. wool garments, which people do not buy often. Even Rav Shrirah Gaon (see below) would agree.

Magen Avraham (10): The Radvaz (1:395) says that Tosfos discusses a rich person, but an Oni blesses. If a very wealthy person bought important Kelim but has no Simchah and they are not important to him, he does not bless.

Mishnah Berurah (24): The Pri Megadim and Derech ha'Chayim conclude that an Oni should not bless on unimportant Kelim.

Rema: The custom is to tell one who wears a new garment 'wear it out and get a new one.' Some say that one should not say so regarding shoes or garments made from animal hide, for a new garment will require killing another animal, and "His mercy is on all His works." This is a very flimsy reason, nevertheless people are particular about this.